Just to let you know that I found a typo for specifying Axis Range. It should be 'chxr' not 'chxp' which is the Axis Postion. Took me awhile to figure out why Axis Range is not doing what it's suppose to be doing.
Line 738: $url .= 'chxp='.implode(',', $this->Graph->axis_ranges).'&';
Anyway, great class! keep up the good work.