diff --git a/core/ViewDataTable/GenerateGraphData/ChartEvolution.php b/core/ViewDataTable/GenerateGraphData/ChartEvolution.php
index e60ddcfdde2f0e06cf55c605f831301c3645e485..a505cff5dc2499354162ef76c808e783d012f182 100644
--- a/core/ViewDataTable/GenerateGraphData/ChartEvolution.php
+++ b/core/ViewDataTable/GenerateGraphData/ChartEvolution.php
@@ -193,7 +193,8 @@ class Piwik_ViewDataTable_GenerateGraphData_ChartEvolution extends Piwik_ViewDat
 		$this->view->setAxisYUnits($yAxisLabelToUnit);		
 		
 		$countGraphElements = $this->dataTable->getRowsCount();
-		$firstDatatable = reset($this->dataTable->getArray());
+		$dataTables = $this->dataTable->getArray();
+		$firstDatatable = reset($dataTables);
 		$period = $firstDatatable->getMetadata('period');
 		
 		$stepSize = $this->getXAxisStepSize($period->getLabel(), $countGraphElements);