fixed for a bunch of graph problems

• 0004463: [graph] pie chart of summary graph per category wrong when viewing allprojects (thraxisp)
• 0004464: [graph] Pie graphs display as error if no issue exist for a specific project (thraxisp)
• 0004511: [graph] Summary Graph "Cumulative by Date" is blank (process expires) (thraxisp)
• 0004507: [graph] No time-line in the summary for cumulative by days (thraxisp)
  (This can be rolled back by backing out the change to graph_api)

git-svn-id: http://mantisbt.svn.sourceforge.net/svnroot/mantisbt/trunk@2940 <a class="text" href="/?p=mantisbt.git;a=object;h=f5dc347c">f5dc347c</a>-c33d-0410-90a0-b07cc1902cb9