X-axis label spacing on a 320x240 graph?

[obhwx]

I am trying to set up graphs to display on a Chumby (http://www.chumby.com/pages/learn_overview) which requires images sized at 320px x 240px; the software they run does not properly resize images down from larger originals. If I set it to 640x480 for example, the displayed image is doubled (two side by side) rather than reduced 50%. When I set up native sized graphs though, the X axis text labels overlap making it look odd.

How can I modify the axis label interval?