# Chord Diagram

A chord diagram is a graphical method of displaying inter-relationships between data radially around a circle. It represents flows or connections between several entities (called nodes) with the relationships between the nodes typically drawn as arcs connecting the data. Here the size of the arc is proportional to the importance of the flow. Chord diagrams are used to visualize data ranging from business use cases to complex scientific data.

### Quick details

What: Discover Interconnections

Why: Display the inter-relationships between data in a circular matrix

## History of Chord Diagram

In geometry, a chord of a circle is a geometric line segment whose endpoints both lie on the circle. Based on this, the chart got this name from the terminology with a corresponding function associated with the chord.  Such a diagram was used in 2007 by the New York Times infographic Close-Ups of the Genome. They are also known as radial network diagrams and may sometimes be referred to as a type of circular layout.

Screenshot from “A Thousand Fibers Connect Us — Wikipedia’s Global Reach”, winning entry of the WikiViz 2011 Data Visualization Challenge. Lines represent readership of different Wikipedia language versions (lower half) from countries (top half)

## When to Use a Chord Diagram?

### 1When you need a simple representation to show interconnections between large datasets

Use chord diagrams to reduce visual complexity which arises when you need to show interconnections between various points through lines, which could make it illegible to read. While a small amount of data could be represented in a circular diagram using straight lines to show the interconnections, a diagram featuring numerous lines could be represented in a chord diagram due to its property of hierarchical edge bundling. Hierarchical Edge Bundling allows to visualize adjacency relations between entities organized in a hierarchy. The idea is to bundle the adjacency edges together to decrease the clutter usually observed in complex networks.

Chord Diagram displaying the number of people migrating from one country to another using data from a  scientific publication from Gui J. Abel.

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### 2When the visual appeal of the chart is high in importance

Use chord diagrams to use a representation format that is aesthetically pleasing, making it a popular choice in the world of data visualization. Chord diagrams are eye-catching and can visualize weighted relationships between several entities. They can also be adapted for several specific situations that slightly modify the output and the way to read them

A bipartite chord Diagram

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### 3When you need to find and compare interrelationships between groups of data

Use chord diagrams when the chart can be made more interactive as it helps in making the chord diagram understandable. One can hover over a specific group to highlight all its connections.

## Types of Chord Diagram

### 1. Bipartite Diagram

Here the nodes are grouped in a few categories where connections go between categories but not within the categories.

### 2. Flow Diagram

These maps can be represented in two ways: One as symmetric arc per pair and the other as two arcs per pair. This is also known as a dependency wheel.

## When Not to Use a Chord Diagram?

### 1When the data set becomes large and generates multiple arc crossings

Do not use Chord Diagrams if over-cluttering makes the figure unreadable. Try to minimize the number of arc crossing and dismiss weak connections. A good way to do so is to draw just a few connections in a first step, before displaying the whole graphic where the group order around the circle is considered important.

### 2When you do not have interactivity embedded and need visual storytelling

Chord diagrams are not straightforward to understand at all. They need plenty of explanation for the audience which can be presented through interactive chord map designs. In those cases, it is possible to break down the graphic, presenting its components progressively. But without interactivity, chord diagrams take more time to comprehend.