BACKEND:

It's about 3000 lines of Crystal code https://crystal-lang.org/ -- it's been an absolute dream to program in. Ruby-like syntax, statically compiled performance. Nice standard library. Would recommend :)

Performance == caching

Local ephemeral filesystem for shortest-term caching.

Redis for shared medium-term caching and locking.

S3-compatible object storage for longest-term caching (for raw data feeds pulled every evening by a cronjob).

FRONTEND:

Bootstrap

uPlot https://github.com/leeoniya/uPlot

Just a sprinkle of inline JS+CSS on the page.