Mirroring Sankey Diagrams for Visual Comparison Tasks

Zana Vosough

, Dietrich Kammer

, Mandy Keck

and Rainer Groh

SAP SE, Germany

Technische Universit

at Dresden, Germany

Keywords:

Information Visualization, Interaction Techniques, Sankey Diagrams, Comparison Tasks, Product Costing.

Abstract:

Complex data sets require suitable information visualizations. With the rapidly increasing amount and com-

plexity of data, the need for suitable interaction techniques to perform various data analyzing tasks is also

growing. Flow diagrams are a powerful tool to understand the structure in hierarchical data sets. In many ap-

plication scenarios, there is a need to quickly understand all facets in the data and compare different versions

to make executive decisions. In order to illustrate our concepts, we selected Product Lifecycle Costing as

application domain in which comparison tasks play an important role. On the one hand, an effective compar-

ison of different versions needs to be visually presented to the user. On the other hand, different dimensions

of the components need to be considered. We propose a mirroring method with the appropriate interaction

techniques based on Sankey diagrams that address both issues.

1 INTRODUCTION

Many real data sets have hierarchical structures.

There has been a large amount of research on dif-

ferent ways to visualize hierarchical data. While in-

formation visualization tools are used widely for un-

derstanding single hierarchies, they are also used for

comparison of two or multiple tree structures. Infor-

mation visualization is crucial for comparison tasks

that are relevant in many domains such as biology,

software systems, medicine, or social science (Mun-

zner et al., 2003; Holten and Van Wijk, 2008; Vrot-

sou et al., 2009; Procter et al., 2010). However, most

research on comparison solutions only provides spe-

ciﬁc strategies that can be applied to individual prob-

lems. Especially for varying data sets with different

sizes and complexities, existing solutions can not be

re-used and new general tools for comparison tasks

are needed.

The presented research is part of a project at SAP

SE with the purpose of ﬁnding new data visualizations

for SAP Product Lifecycle Costing (PLC), which is

a standard business intelligence (BI) solution. Simi-

lar to other BI applications, the user interface of the

application is a spreadsheet environment. Although

numerous visualization tools have been built helping

analysts to extract meaning and understand relation-

ships in their data sets, there is still a lack of research

in the visual presentation. The available visualization

methods do not possess the expressive power needed

to match the complexity of current data and analytic

questions in BI applications.

This shows the demand to introduce visualizations

that support these analyses by providing an interactive

graphical access to those aspects of lifecycle costing

that can hardly be captured in a tabular display, such

as costing dependencies, the hierarchical compound-

ing of costs, or version comparison tasks.

Comparison of two individual hierarchies or com-

parison of multivariate or dynamic graphs (Andrews

et al., 2009) are critical tasks for users. This paper

proposes a solution that can be used for comparing a

graph using two different structures or a graph from

two points in time. We ﬁrst describe the research con-

text and the problem domain. Next, the visualization

concept that can be applied to two task types from

the problem domain is presented. Lastly, we conclude

with a set of guidelines for future research directions.

2 RELATED WORK

Different taxonomies are suggested for comparison

solutions, for instance, Graham and Kennedy inves-

tigated suitable task areas for different tree visualiza-

tions, varying from single trees to pair trees and mul-

tiple trees (Graham and Kennedy, 2010). Pang et al.

reported the importance of comparative visualization

for ﬂuid dynamics data and some possible solutions

Vosough, Z., Kammer, D., Keck, M. and Groh, R.

Mirroring Sankey Diagrams for Visual Comparison Tasks.

DOI: 10.5220/0006651203490355

In Proceedings of the 13th International Joint Conference on Computer Vision, Imaging and Computer Graphics Theory and Applications (VISIGRAPP 2018) - Volume 3: IVAPP, pages

349-355

ISBN: 978-989-758-289-9

349

(Verma and Pang, 2004). Gleicher et al. propose a

general taxonomy based on a design strategy for vi-

sual comparison that categorizes all designs of com-

parative visualization in three basic categories, which

can also be combined (Gleicher et al., 2011). Our

focus in this paper is on the comparison of two hier-

archical information structures using a juxtaposition

approach.

There are three common ways to represent tree

structures: explicit representations such as node-link

representations (Battista et al., 1998), Radial trees

(Battista et al., 1998), or Sankey Diagrams (Schmidt,

2008) that represent relations between nodes by lines

or ribbons connecting them. Implicit or space-ﬁlling

representations use parent nodes enclosing their child

nodes such as TreeMap (Johnson and Shneiderman,

1991) or Icicle Plot (Kruskal and Landwehr, 1983).

Finally, hybrid approaches combine explicit and im-

plicit representations like Elastic hierarchies (Zhao

et al., 2005) or SHriMP (Storey and Muller, 1995).

Some of the above mentioned visualization solu-

tions are used in different ways for comparing hier-

archical structures. The task of comparing multiple

trees has been solved either by using these common

tree representations and extending them in a way that

they can be used not only for exploration tasks but

also for comparing two or more tree structures or by

designing completely new visualizations for speciﬁc

scenarios like ActiviTree (Vrotsou et al., 2009) and

Multiple Trees through DAG Representations (Gra-

ham and Kennedy, 2007).

There are different visualization solutions avail-

able to extend the explicit presentations of trees, for

instance, Contrast Treemaps (Tu and Shen, 2007) or

Generalized Treemaps (Vliegen et al., 2006) extend

Treemaps for visual comparison tasks. Furthermore,

other approaches are designed with implicit represen-

tations such as Hierarchical Edge Bundles that show

the relationships between trees by extending Icicle

Plots (Holten, 2006).

In this paper, we propose a new solution to extend

another explicit representation, Sankey Diagrams for

comparison of two tree structures.

3 SCENARIOS

In this section, we describe the research context and

select two comparison tasks that are addressed in this

contribution.

3.1 Background

SAP Product Lifecycle Costing (PLC) for SAP S/4

HANA is a solution to calculate costs for new prod-

ucts and generate quotations. The software helps

to quickly identify cost drivers and to easily simu-

late and compare alternatives. PLC was developed in

close collaboration with more than 30 co-innovation

customers over a period of four years. A research

project at SAP SE seeks to ﬁnd new data visualiza-

tions for this standard BI solution (Vosough et al.,

2017a). The current interface is a spreadsheet en-

vironment, which is typical for conducting lifecycle

cost analyses in business intelligence applications.

Different user studies on current visualization

practices, customer visualization tasks, and character-

istics of the visualized data were conducted to under-

stand the user requirements for a new data visualiza-

tion. After two rounds of group discussions with 30

customers from 16 companies, four main tasks were

prioritized.

• T1: Recognition of deviation between calculation

and deﬁned cost target and identiﬁcation of as-

semblies that are above or below target cost.

• T2: Identiﬁcation of the main cost drivers by

comparing multiple cost calculations with each

other.

• T3: Determination of incomplete or inconsistent

cost calculations.

• T4: Assessment of the reliability of the overall

cost calculated from price sources with different

conﬁdence levels.

During the requirement engineering phase, two

data visualizations were introduced to support these

tasks: Treemaps and Sankey diagrams. Both pro-

totypes were evaluated in an informal setting at two

customer workshops and Sankey diagrams were pre-

ferred by customers of the project (see (Vosough

et al., 2017a)). Building upon this work, this re-

search extends Sankey diagrams to solve more com-

plex tasks. The proposed mirroring techniques ad-

dress T1 and T2 while T3 and T4 are the concern of

further research.

3.2 Problem Statement

As described in the previous section, one of the chal-

lenges that customers of the project are dealing with,

is to ﬁnd the main cost drivers by comparing multiple

cost calculations with each other. Users need to gauge

the impact of adding or removing individual items or

IVAPP 2018 - International Conference on Information Visualization Theory and Applications

350

Table 1: Industrial pump with different dimensions of components.

Structure 1 Structure 2

Level Item Name Cost Country Company Code

1 Pump P-100 20.200 - -

2 Casing 8000 - -

3 TCD (setup) 826 USA #CC2

3 TCD (machine) 1888 USA #CC1

3 Slug for casing 921 Germany #CC11

2 Pick-pick list 1496 - -

3 Turn shaft-speciﬁcation 621 USA #CC2

assemblies on the overall costs. This challenge con-

cerning the costing data can be addressed by two in-

dividual tasks.

3.2.1 Structure Comparison (T1)

The costing data structure is hierarchical and multi-

dimensional, since the overall product cost is the sum

of the sub-part costs, raw materials, and associated ac-

tivities. In addition, the total cost can be broken down

based on different dimensions such as cost component

split, material types, countries of origin, maturity lev-

els, cost centers, plans, or weight. Table 1 shows se-

lected data from an industrial pump and its different

dimensions such as component split and country of

origin.

Complete data sets also provide company codes

for each country and cannot be handled appropriately

by the current spreadsheet user interface. Showing

the cost impacts of each company code or country

along with the whole data structure is not possible,

but would be extremely helpful for customers in or-

der to leverage the information contained in their data

structures.

3.2.2 Version Comparison (T2)

One product calculation consists of several versions.

These calculation versions (CV) are used to take dif-

ferent scenarios into account, so that the cost devel-

opment of a product can be projected into the fu-

ture and factored into the analysis. Cost of an item

changes over time for different reasons, such as im-

pact of learning curves, currency ﬂuctuation, gover-

nance laws, commodity price changes, or inﬂation

rates. Table 2 shows the same example from Table 1

with the ﬁrst structure, but from two different points

in time. It represents how the cost of casing and pick-

pick list in the second level change based on the cost

of their sub-items. Moreover, new items might be

added or removed from the cost structure.

The next section describes our visualization con-

cept that supports end-users to perceive and under-

Table 2: Two versions and associated costs of an industrial

pump.

Version 1 Version 2

Level Item Name Cost Cost

1 Pump P-100 20.200 19.400

2 Casing 8000 7.300

3 TCD (setup) 826 826

3 TCD (machine) 1888 1188

3 Slug for casing 921 921

2 Pick-pick list 1496 1596

3 Mill groove - 190

stand these two characteristics of the costing data

more effectively.

4 VISUALIZATION DESIGN

Based on the customer feedback reported above, our

research focuses on using Sankey diagrams to visual-

ize costing data. Sankey diagrams are ﬂow diagrams

that emphasize quantities in a data set (Riehmann

et al., 2005). The thickness of the links (ﬂows) be-

tween the items (rectangular nodes) shows their quan-

tity, which corresponds to the cost of a component in

our solution. Hence, on the very right side, singular

components are displayed and their cost is subsumed

in component groups to the left. Sankey diagrams

can be created with multiple levels of connections and

facilitate ﬁnding items that dominantly contribute to

the total product cost. Sankey diagrams are particu-

larly suitable for understanding how a data structure

is composed and for understanding relationships be-

tween elements (Schmidt, 2008).

In order to solve the two main user tasks out-

lined earlier, we considered different comparison so-

lutions that leverage the strength of Sankey diagrams

to show many-to-many mappings between two do-

mains or multiple paths through a set of stages. Our

solution is designed to keep the parent nodes or the

leaf nodes of two costing structures in the middle and

then visualize the complete structures on both sides.

Mirroring Sankey Diagrams for Visual Comparison Tasks

351

Figure 1: Visualizing a costing data structure with two facades with Sankey diagrams. The data is structured on left side

(green) based on cost component split and on the right side (blue) based on location. Parents node of the selected component

are shown by highlighted bars.

In the following, we explain each solution in more de-

tails. All visualization prototypes were implemented

in JavaScript using jQuery

and the visualization li-

brary D3

and the color of choice for dimensions are

from the ColorBrewer website (Harrower and Brewer,

2003).

4.1 Structure Comparison Task (T1)

The ﬁrst task addresses the problem that each cost cal-

culation can be composed based on different criteria,

which results in different hierarchical data structures.

In the typically used spreadsheets, hidden relation-

ships between different structures can not be captured.

The challenge is to ﬁnd a visualization that can help

with extracting this information.

Finding an appropriate visualization for a single

hierarchical graph is not an easy task, however, when

comparing two graphs this task becomes even more

difﬁcult. One problem is that users need to per-

ceive the relationship both within one graph and be-

tween two graphs. Gleicher describes three main tax-

onomies of visual design for comparison: juxtaposi-

tion, superposition and explicit encoding. Also, the

three designs can be combined to create hybrid solu-

tions that beneﬁt from features of two solutions (Gle-

icher et al., 2011). Juxtaposition is a simple solu-

tion that puts different objects next to each other. It

is a simple approach but not always efﬁcient as it re-

https://jquery.com/, retrieved on 13.03.2017

https://d3js.org/, retrieved on 13.03.2017

quires more space and relies on the user’s memory to

build the connection between objects. Superposition

works by overlaying objects on top of each other, and

explicit encoding computes and directly shows rela-

tionships between objects.

Superposition is not a practical solution for

Sankey diagrams, since the data structures are totally

different for each scenario and overlaying prevents

users from seeing relationship both within and be-

tween two graphs. A solution to avoid visual clutter-

ing is to show only differences between two super-

imposed diagrams, however, showing the hierarchy

is important for costing data and with this approach

information is lost. Moreover, the leaf nodes on the

costing graphs stay consistent and only the items in

the middle levels and subsequently the parent node

change.

We designed a Sankey diagram that keeps the

leaf nodes in the middle and placed two structures

on both sides (mirroring around leaf items). With

this solution, both the tree structures and the rela-

tionship between different items are preserved. How-

ever, we save space by visualizing the leaf nodes that

are the dominant items in the costing data structure

only once. In addition, the problem of too much in-

formation to remember for users is mitigated, since

the items have connections and recognizing relation-

ships between them becomes easier. This is further

facilitated by adding supportive interaction methods.

Figure 1 shows one example of an industrial pump

that has two dimensions, cost component split and lo-

cation. The leaf nodes are represented in the middle

IVAPP 2018 - International Conference on Information Visualization Theory and Applications

352

Figure 2: Visualizing two cost calculation versions with Sankey diagrams. Version 1 is shown on the left side (orange), and

version 2 on the right side (green). It shows a comparison between the items on, a) the ﬁrst level, b) the second level, and c)

the third level, close to each other along with their sub-items.

with a different color (orange) in order to better distin-

guish between component split structures on the left

(green) and location on the right (blue).

Interaction techniques are a pivotal tool to en-

hance visual comparison. One common solution in

the domain of information visualization is visual ﬁl-

tering using highlighting, described by Becker et al.

as brushing with a special color to paint an object

(Becker and Cleveland, 1987). In order to assist the

comparison task, we use common brushing interac-

tion to establish connections between related compo-

nents. By hovering an item with the mouse, the cor-

responding parents are highlighted and it is easily ob-

servable to which components this item belongs and

where it originates. Furthermore, the relationships be-

tween the middle levels can be inspected by hovering

with the mouse. For instance, when hovering the cas-

ing item (second level left), the companies and coun-

tries that this component mainly originates from are

highlighted. These interaction techniques play an im-

portant role to enhance the visual understanding of

hidden relationships in costing data.

4.2 Version Comparison Task (T2)

As described in the ”Problem Statement”, the sec-

ond task is about analyzing the changes in structure

within the costing data over time. This comparison

task has different data characteristics. In this task,

two changes can occur, the data structure stays con-

sistent and the item costs change slightly or few items

can be added or removed in the data structures. Beck

et al. distinguish between animation and timeline for

dynamic graph visualization techniques (Beck et al.,

2014). Within the timeline category, juxtaposed, su-

perimposed and integrated approaches can be consid-

ered for node-link structures. In the integrated ap-

proaches, the graphs are interlinked and cannot be

separated, which is not a suitable solution for our

problem. First, superimposing for comparing two

calculated versions was considered. The structures

of two version were stacked on top of each other

with different color codes. From the result we con-

cluded that superposition is not suitable for ribbon

ﬂows, since the visualization becomes extremely clut-

tered and unintelligible. Another possible solution is

switching between two versions of the same diagram.

A fast prototype was implemented to switch in a cer-

tain interval between two images of Sankey diagrams.

The arrangement of the items proved to be a problem

in this approach. Sankey diagrams use different tech-

niques to arrange the items. For instance, in our solu-

tion items are arranged based on their size (cost), but

this can change in different versions as the item val-

ues change. Although research shows that timeline

approaches provide better analysis instead of anima-

tions (Tversky et al., 2002).

Finally, the solution was designed by placing two

data structures (versions) next to each other, since

juxtaposition was the preferred solution. In contrast

to the structure comparison task, the parent item is

placed in the middle and the sub-items of both struc-

tures are arranged around them (mirroring around par-

ent items). The rationale behind this design is that

users usually want to identify the differences between

the total costs at the ﬁrst glance. By putting the par-

ent nodes (bars) close to each other, this comparison

becomes easier (see Figure 2).

Mirroring Sankey Diagrams for Visual Comparison Tasks

353

As the values of other items cannot be easily com-

pared, especially when the differences are small, ap-

plying proper data abstraction is necessary. There are

different solutions available to visualize comparison

of complex data that decrease the complexity by ab-

stracting the data (Amenta and Klingner, 2002). In

this work, for a more accurate comparison, the levels

of interest can be clicked and thus placed next to each

other. The resulting image is similar to a simple bar

chart containing two bars, which is easier and faster to

interpret. By double clicking on the levels in the mid-

dle, the graph is unfolded and switches to the initial

view. Although this strategy follows the juxtaposi-

tion strategy, due to the interaction with the different

views, it still conveys the feeling that the images are

overlaid (Roberts, 2004).

Figure 2 shows an example of two versions of a

cost calculation for an industrial pump from different

points in time. The ﬁrst version represents the items

on the left side in orange and the second version on

the right side in green. In the ﬁrst picture (a) all items

are presented and it can be immediately seen that the

total cost decreases over time. By hovering each item

with the mouse, additional information such as the ex-

act price and relative price are shown in a tooltip. By

clicking on any item on the second level, the graph is

folded and items on the second level are moved next

to each other (b). Subsequently, by clicking on any

item from the third level, those items are moved next

to each other (c). By double clicking anywhere on

the screen, the visualization switches back to the ﬁrst

view and shows all items again (a).

4.3 Discussion and Limitations

Although the concepts and design decisions have

been made based on customer interviews and work-

shops, the presented mirroring techniques have not

yet been evaluated in a formal setting. Since the

presented research is still work-in-progress, different

variations could be compared for their effectiveness in

a user study. There are some limitations with regard

to the readability of the labels on the nodes when the

number of components becomes exceedingly high.

In that case, zooming or details-on-demand could be

used.

Another key challenge in comparison tasks is

dealing with scalability. It involves both the complex-

ity of the tree and the number of trees to be compared.

These could be considered as one of the limitation

of our current solution to represent complex trees or

more than two hierarchy structures.

5 CONCLUSIONS AND FUTURE

WORK

In this paper, we presented two Sankey based visu-

alizations along with an interaction concept to facil-

itate two speciﬁc comparison tasks. Both visualiza-

tions are based on a juxtaposition approach and con-

nect two Sankey diagrams by putting either the parent

or leaf nodes in the center of the visualization.

We focused on methods to compare two complex

graphs, but this work can be extended to multiple

graphs, since both scenarios contain more than two di-

mensions. One approach would be connecting many

Sankey diagrams on the horizontal axis and pan left

and right to see the different versions. Another idea

to enhance the visual perception is to add more vi-

sual components to make the comparison tasks easier.

For instance, one approach is to use color codes to

indicate the changes in the data structure over time.

When item costs increase, this can be indicated by

a red color and when it decreases by green to help

the user with processing the positive and negative

changes. This visual feature can be applied both on

ribbon ﬂows or bars in Sankey diagrams (Vosough

et al., 2016; Vosough et al., 2017b).

ACKNOWLEDGEMENTS

We thank all members and customers of SAP Product

Lifecycle Costing for contributing their experiences

and sharing input on different parts of this research

with us.

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