See http://forum.piwik.org/read.php?5,78387; Reproduced in Version 1.5 with FF5 and Chrome 12 on Piwik Demo and my own instance of 1.5.
Another report in: #2525
The one where a graph does not appear is the one showing a percentage of "returning visits have bounced".
 fixed this bug too. BONUS!
Actually, I think was fixed by . Still good.