graphs in the reports are not showing

Hi all:

We have generated several reports in Youtrack 6.0 Build 12463 where we have selected "presentation: Grouped Bars". When I run the report I can see the numbers but the Bars never shown.  My you track in installed under windows. I have tried with several reports, several projects, several browsers but never work.

Please could you give some idea why the graphs are not showing?

Thanks in advance.

Attached a screenshot