tiny-plot: improve tick labels in scatter plots that occupy very small or very large ranges

[AlexTate]

Tick labels in scatter plots are improperly applied when an axis has a very small or very large range. Labels in small ranges are sparse and irregular, and labels in large ranges become crowded.

This undesirable behavior results from:

1. A fixed n for "every other n" tick labeling. Currently it is fixed at n = 3 which makes labeling inflexible at extreme ranges
2. There is a rounding bug in the routine that determines major tick locations. Under certain conditions it results in ticks being placed outside of the plot space, which is an issue for downstream routines that mitigate label crowding at the lower and upper ends of each axis. This is most apparent when the axis range is very small.