Render an X and an Y axis at cursor position in sample view when sample-drawing pencil is enabled.
This is like a ruler.

Serves two purposes.

1. Helps you eye amplitudes faster when looking at the other cycles of the waveform.
2. Is warning signal that you left drawing mode on. Too many times I was dingus and accidentally drawn spikes into sample. Derp.

Adendum:
We have a measurement bar on top of sample. What if we add a second vertical axis, adjustable to be in front or behind cursor position by X amount?
This improves eyeing even more.
Now the ruler has become a caliper!