Howdy, Is there an easy way to turn off the minor gridlines
> in a (semi)log plot? I just want the ones that are at the
> powers of ten.
Technically, there are not major and minor ticks for the default log
tick locator and formatter, there are only major ticks and the decades
are labeled. It is on my list of things to do to break these into
major and minor ticks. At that point it will be fairly easy to turn
on and off the grids separately for the major and minor ticks.
If you want to look into this, the relevant code is LogLocator and
LogFormatter in matplotlib.ticker.py. It should be fairly easy to
modify this code to support major and minor ticks separately.