If you dig elixir's LiveView, you can do something similar:

https://news.ycombinator.com/item?id=36371907

Any clear reason to use this over chartkick?

README: “The idea of this gem was inspired by Chartkick gem which is great and allows you to build charts very quickly. It works best in cooperation with groupdate gem. Unfortunatelly it's missing many needed types of charts or other customization options.”

if one wants additional chart types, this is a decent option

The calendar chart looks fun, I wonder how customisable it is in terms of colour or tooltip highlights?

Oh, it's a library for doing random charts using rails. Not a library for doing rail charts.