# Filled Chord Diagram in R using Plotly

In this post we’ll create a Filled Chord Diagram using plotly. The post is inspired by Plotly’s Python documentation.

## Install / update packages

Just to ensure we are working with the latest dev version of plotly.

## Dataset

The dataset we’ll use consists of the number of comments a person made on her own facebook posts as well as the number of comments on her friend’s facebook posts.

## Global settings

These are some plot related settings which we can setup right now. These settings will be fed to `plot_ly()` later on. Also, having these aesthetic settings accessible now will make it easier for us to make changes later on.

## Creating the Ideogram

We’ll first create the ideogram. The ideogram which is essentially a set of sectors plotted on a unit circle which’ll represent the rowsums of the dataset i.e. the total number of comments made by each person (to themselves and their friends). We’ll first create some helper functions – `toAngular()` to map a vector of numeric values onto the unit circle using cumulative sums essentially creating sectors and `addGaps()` which’ll create some space between each sector for aesthetic purposes.

Now that the functions are defined we need to create the ideogram. We do so by:

1. Creating an outer ring which’ll lie on the unit circle
2. Creating an inner ring which lie inside the outer circle (radius is defined in the settings section above)
3. Creating an SVG path for each sector bounded by the outer and inner circles and the end points of each gap

You should now have something similar to this:

## Creating Chords

The following set of code snippets essentially do this:

1. Divide each sector on the ideogram into sub – sectors based on the number of comments made by each person i.e. traverse the dataset row-wise and map the numeric vectors in each row onto the associated sector (not the unit circle). Example: Emma has a total of 65 comments amongst herself and her friends. The sector corresponding to Emma’s 65 comments needs to be divided into further sectors based on the 16, 3, 28, 0 and 18 comments.
2. Find the four points that bind each chord. A chord is a planar shape that is bound by two bezier curves and two circular arcs. For each bezier curve the control point is generated by finding the mean of the angluar coordinates of the end points.
3. Fill each chord (ribbon) with the appropriate color. Example: Emma made 18 comments on Sophia’s posts but Sophia made 23 comments on Emma’s posts. Since Sophia made more comments, the chord depicting the interaction betweem Emma and Sophia is colored using the same color that is used for coloring Sophia’s sector on the ideogram.
4. Create SVG shapes for each chord and then plot using `plot_ly()`

Note that the ordering of the endpoints of bezier curves and circular arcs is tricky and was done based on trial and error.

You should now have something like this:

I am sure there are more efficient ways of going about this but hopefully you found this post helpful. Here are some additional resources to look at: 