matplotlib question

[asdf]

basically I need to plot a graph of data vs time. However when i use
matplotlib the hr:min tick marks come out very close together and
appear jumbled. So 12:00 comes out very close to 12:30 for example.
There are two things I would like to do. First, is to increase
the horizontal dimension of the graph. So basically increase the
horizontal number of pixels. The data will always be from 
midnight to midnight it's just that i want it stretched out
more horizontally. Also, how do i specify that i only want hourly
tickmarks. So under the x-axis i only want to see 12:00 1:00 etc.

thanks