Extending fftsink2 for static object

Request for comments from the plot.py guru(s) before diving in:

I’d like to extent the fftsink2 (fftsink_nongl on my street) plot canvas
to include static objects such as a stationary reticule line, info-text,
boxes, etc.

I’m guessing the entire canvas is redrawn with every fft data event so
it would need some kind of data structure to merge with the fft points.

I’ve hacked plot.py to make a black background with green fft and an
orange grid for an attractive look.