Add endpoint to Leetcode activity chart

[volyx]

Maybe use this library for that - https://frappe.io/charts what do you think? Or maybe it is possible to build a chart without libraries

Frappe chart 'A Month-wise Heatmap'

[JeremyTsaii]

That library looks promising. This is a great idea. I will be a bit busy with school for now, but feel free to open a PR!