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.

What version of LS are you using?

I am using Version 2.05+ Build 141210. Thanks!

Well, your version is fairly old (almost 2 years). There have been a few changes (bug fixes, improvements, etc) in Limesurvey since then.

I think it doesn't make sense to try to find out if this is a bug or not, because it might have been fixed long ago. So the first thing you should do is to update to the latest version of the 2.0x branch.

You could also update to 2.5x branch, but this is quite different. So before you update your installation to this, you should install a test version and check it thoroughly if it works for you. Only then update to 2.5x branch.

Or maybe what you could do is check on the demo here if it works. This should run a fairly up to date 2.5x branch version of LS (I just see that it is 2.51.4, which isn't actually very old, but there have been a few changes).