CSNext report does't export CHART properly to the excel file

Hello,
I have noticed one significant problem. Reports built by BIRT in RTC client when they are published is possible to see the chart.
When it is exported to the EXCEL file chart in BLANK.
I see that correct chart is exported to the excel but instead of CHART I just see EMPTY square.

Chart in report is as PNG file, in other case it wouldn't be visible on WEB in RTC.

Could you give a clue what is the reason of missing chart in excel file?


Regards,
Marta