api/ui: smart downsampling/smoothing to combat noise

[flyingmutant]

Graph like this:

is not really that much more useful than this:

but is much harder to read. We want to show noise where it is non-uniform (and shows us some interesting property of data); uniform noise is just noise.

See:

• uPlot smoothing demos
• LTTB downsampling
• ASAP smoothing, demo
• https://en.wikipedia.org/wiki/Edge-preserving_smoothing

We don't want to smooth too much, because this lowers reaction speed for sudden spikes.