GSoC: Legend module

My Name is Eduardo Gonzalo Espinoza Carreon and during this summer I developed the new Legend Module for Gephi, with the mentoring of Eduardo Ramos and Sébastien Heymann. This article will give you an overview of the work done.

Problem statement

Currently Gephi offers the possibility of visualizing graphs, but what about legends? Legends provide basic and extra information related to the graph and they are useful when interpreting any kind of network map. If a person is not familiar with the content of a graph, missing or wrong legends could lead to misleading interpretations and sometimes wrong decisions. When a visualization is used by multiple people for discussing, analyzing or communicating data, legends are of great importance.

For instance, the following graph represents the coappearance of characters in the novel Les Miserables. After performing a visual analysis we could only conclude that the graph has 9 groups. This is probably a little of the information the creator wanted to transmit. The graph has no information related to the number of nodes explored, or what the groups represent and how many elements each group has, etc.

A current workaround to solve this problem is to export the graph as an image, and then manually add the legends using Inkscape, Adobe Illustrator or another graphics editor. However this task is time-consuming and can be automated. The new Legend Module proposes a solution to this problem.

Solution

We propose an extension to the Preview module for generating legend items. The following legend items are available: Table, Text, Image, Groups and Description. They can be added using the Legend Manager, which is shown in a new tab under the Preview Settings:

After selecting a type of legend, the user chooses a sub-type builder, e.g. “Table” > “Partition interaction table”, or “Top 10 nodes with greatest degree”, as shown in the following figure:

When a new Legend item is added, it is displayed in the list of active legend items, where the user can edit its properties. The user can also edit its label and assign a user-friendly name to remember the content of the legend easily.

Every item has a set of common properties: label, position, width, height, background color and border, title, description; and also each type of item has its own properties and data. The values of those properties are editable through a Property Editor like the one used in the preview settings.

Some properties like scale and translation can be modified using the mouse like most of the graphic design applications. All legend items are designed with a smart way of autoresize. It’s not the common scale feature, e.g. if the text included in the Text Item is bigger than the size assigned, then the Text Renderer overrides the text font defined by the user and decreases the font size until the text is able to fit in the specified dimensions. The results of this feature are shown in the next figure:

Workflow

The legend builder retrieves the graph data (partitions, node labels, edge labels, etc) and creates a new Legend item for each of them. Then a legend renderer makes use of these information, plus the properties set by the user, to render the Legend item to the specified target: PNG, PDF or SVG.

For developers

The renderers can be extended. For instance, the default Group Renderer is:

Using external libraries like JFreeChart, we can extend it to create a Pie Chart Renderer like as follows:

Other types of items can be created by combining other available Legend Items or by extending Legend Item, Legend Item Builder and Legend Item Renderer.

The Legend Module also provides a save/load feature. So you can save your legends for future editing.

Limitations

Currently there are some limitations like selecting a specific renderer for each type of item, and also exporting legends to SVG format is not done automatically like PNG and PDF, e.g. Exporting an Image (they will be embedded in the SVG file).

Conclusions

I would like to thank Eduardo Ramos and Sébastien Heymann for their support and feedback, which was critical during the development of this new module. The Legend module will be available as core feature in next Gephi release.

This GSoC was a great opportunity to learn and it also represents my first important contribution to the open-source community.