Add support for a fixed-width (either in time or number of samples) plot

[vkottler]

This helps ensure that plot performance doesn't plummet as samples accumulate. You can't currently leave the GUI running because of this

[vkottler]

Maybe 10-20s as a start is good? Actually, since we're running the UI at a fixed frequency (which is sampling the state of the underlying program), maybe a sample depth (based on the UI rate) makes sense.

Err... even if the human interface to set the threshold is in seconds, under the hood we can compute sample depth.

[vkottler]

I think a good "default" behavior should be to set a sane cap on the number of max samples at any given time (a stateful member of the TUI task), maybe like 1024?

That way the plot "grows" until it becomes saturated with samples

this should be configurable via the config file, too

[vkottler]

Implemented here: #45.

About 64 points max on the plot looks pretty good with a full-screen app open.