pyRevit charts module is pyrevit.coreutils.charts. This is the module that the output window interacts with to create the charts.
The charts module provides the chart object and handles the creation of datasets. The first thing it does when drawing the graph is to create a html <canvas> element and assign a unique id to it:
<canvas id="chart123456"></canvas>
Then it parses the input data and creates a JSON representation of the data. The JSON string (json_data) will be inserted into a template javascript. This javascript creates a Chart object from the Chart.js library:
var ctx = document.getElementById('{}').getContext('2d');
var chart = new Chart(ctx, json_data);
and finally, the pyRevit chart object, injects this dynamically created javascript into the <head> of the output window WebBrowser component:
output.inject_script(js_code)