further refinement of 0004808: Not exactly true data shown in graphs
used a more complex SQL query to reduce the runtime
rounded the data to days, rather than seconds

git-svn-id: http://mantisbt.svn.sourceforge.net/svnroot/mantisbt/trunk@3149 <a class="text" href="/?p=mantisbt.git;a=object;h=f5dc347c">f5dc347c</a>-c33d-0410-90a0-b07cc1902cb9