has vertical grid lines ONLY on the minor ticks, which
> looks silly. How do I get regularly spaced grid lines
> here?
This is a consequence of the fact that the ticker skips minor ticks
which coincide with major ticks. I am not sure this is the right
behavior, but was implemented to avoid other problems (overlapping
gridlines, for example, with anti-aliasing produces undesirable
results). All you need to do is turn both major and minor grids on
ax.xaxis.grid(True, which="minor")
ax.xaxis.grid(True, which="major")
Should work....
JDH