For the charts he's using chart-js with a chartjs-plugin-rough [1] to give it the hand-drawn look. Chardin Js for the overlays [2] and it looks like the rest of the graphics are created by hand using Sodipod or Inkscape [3].
I liked the freeze/scroll UX as well. Not sure how screen-reader friendly it is, but some of it (especially the click to see axes marked) is quite nice.
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[1] https://nagix.github.io/chartjs-plugin-rough/
[2] https://heelhook.github.io/chardin.js/
[3] https://inkscape.org/