Use the drawScale( $Format ) function.
You can set several parameters in the $Format array.
http://wiki.pchart.net/doc.doc.draw.scale.htmlYou want to use the 'LabelSkip' parameter.
$MyGraph->drawScale( array('LabelSkip' => 5) );
Unfortunately, I couldn't get the label skipping option to draw a graph at regular intervals of 5.
Instead of the desired: 0,5,10,15,20
I get: 0,4,9,14,19
I don't know if it is because the LabelSkip option seems to start counting at zero or what, or maybe there is a bug.