Sounds like you have encountered this issue:
For More Information see: PDF Output or printing a mediafield PDF looks bad
This is one of many acknowledged bugs that can be found in the Known Bug List thread here in the Report an Issue section of the forum.
It can also be downloaded as a database file from: https://www.dropbox.com/s/jt09b82i0xijbu3/FMP%20Bugs.zip
Thanks for your post!
Allow me to explain more details:
1. This issue just occurred when I use Line Chart, other types are OK!
2. I have tried on FM12v3, it's the same!
P/S: I use Windows 8 Pro x64 / Windows 7 Enterprise x64
Given the type of chart, the issue may be more noticeable with line charts. In any case, you are welcome to use Report an issue to report this issue in order to see what the TS personnel think is the cause of this issue.