workaround:

— frontends/php/include/classes/class.cchart.php.orig 2012-09-13 00:55:14.000000000 +0400
+++ frontends/php/include/classes/class.cchart.php 2012-09-13 00:53:53.000000000 +0400
@@ -1666,7 +1666,7 @@
*/
protected function getYStepMarkerValueOffset($yAxis, $stepNumber) {
$step = $this->gridStep[$yAxis];

• $minY = $this->m_minY[$yAxis];
  + $minY = abs($this->m_minY[$yAxis]);

$offset = 0;
if ($stepNumber > 0 && $minY) {

P.S. also not properly calculated min/max values ??by fixing one of the boundaries