setting grid density

Hello, I'm new to matplotlib and this list.
Congratulation to the developers of this great package, I've been looking
a long while for a good quality plotting package.

I would like to plot time in hours on the x axis, with one minor
grid per 15 minutes and one major grid per hour, but I can only see
10 fixed grids on all tutorial and documentation.

How can I configure that?


Philippe Strauss