Cannot display label in the chart on statistics survey results - by: letty

When I try to view the survey responses by clicking the "Get statistics from these responses" menu item under "Responses & Statistics" button, it displays the chart successfully for both English and Chinese characters. However, label in the chart cannot be displayed for Chinese characters while label can be displayed for English characters. I checked that the "fireflysung.ttf" is already put under the "font directory" folder.

You may refer to the attachments:
1) Chart display Chinese characters.JPG - Chart displayed without label for Chinese characters
2) Chart display English characters.JPG - Chart displayed with label for English characters

Is there anything that I'm missing to get the labels display in the chart?

Thanks in advance for your answers and help.