This is what I believe the problem to be.....
One of the columns on the graph is considerably bigger than all of the others.
Fix...
find stats_sales_report.php in admin folder.... then find in the code....
//define variables for graph
if ($report->size > 1) {
$scale_x = ($sum / $report->size) / 4;
$scale_y = $scale_x + 50;
$scale_z = $scale_y / 100;
$scale = round($scale_z) * 100;
Change 50 to a greater number, I changed mine to 100.
It's a quick fix, and really it's a bug that needs rectifying.