Refs #2159

 * Fixing RowEvolution popover click on selector: Warning:</strong> <i>array_keys() expects parameter 1 to be array, null given</i>
 * Changing default monthly to show 2 years history
 * Simplifying code of getXAxisStepSize() since it seemed too complicated ;)
 * removing incorrect title which was always "Showing evolution over last 30 $period"

git-svn-id: http://dev.piwik.org/svn/trunk@7075 59fd770c-687e-43c8-a1e3-f5a4ff64c105