After two days I found the problem :
Demo with my case :
HTML :
<div class="graph">
<div id="chart" style="width: 535px; height:275px;"><!-- chart render here --></div>
</div>
and CSS :
.graph {
position: relative;
}
Do not put the css position to "relative" and the chart will be correct on your pdf. Check on your code if the parent div is not on "relative".