Can’t find mention of it before, but while looking for time interval stuff for a personal project I cam across Epoch.
https://epochjs.github.io/epoch/
No idea if there is any mileage in it for emoncms or apps, but it looks cute (and “simple” for JS)
Looks nice!
Another one that’s nice is highcharts.
https://www.highcharts.com/demo
That looks good, with lots and lots of flexibility, but appears to be a non-free product, while Epoch is MIT licensed.
Highcharts is free for personal use.