Make out-of-range values visible at the bottom of the plot

[argilo]

This is an alternative to #1296.

Instead of removing clamping, we could correct the clamping calculation so that out-of-range values are visible at the lower edge of the plot the same way as they are at the top.

Before:

After:

[willcode]

This had no effect for me, but replacing 1.0f with 2.0f did work. There may be some combination of m_DPR and rounding affecting this. I did not find the magic constant in a few tries.

[argilo]

Oops, I guess I'll have to do a deeper dive into the calculations to see what's going on.