odd axis behavior

When I do the following, the plot is doing something weird. It fills up a
triangle area between first and second points, after the axis command:

n = 5000.0

This only happens at a very narrow window of boundary conditions.
If n is smaller (e.g. 4000) or if Y range is larger (e.g. set the min at -50.2
instead of -50.1), it becomes normal.

JC Hsu