Around May 2012 (Piwik version 1.6 if I'm right) Internet Explorer 9 users complained that the pie and bar graphs were not visible anymore. It seemed that Compatibilty View (which causes IE9 to run as IE7) solved the problem.
In Piwik version 1.10 this trick didn't work anymore. The pie and bar graphs were not visible anymore in any IE. It seems to be an IE security setting not to trust anything in an iframe.
The issue is solved for IE version 8 and higher by adding a line in the parent page:
<meta http-equiv="X-UA-Compatible" content="IE=edge,chrome=IE8" />
Details can be found in the forum: http://forum.piwik.org/read.php?2,88695
Keywords: widget explorer iframe
In 8e0423e: Fixes #3707 fixing widget under IE9
In 3d894e1: Fixes #3707 final fix and tested on browsershots.org