fix invalid encoding error at Network Graph

Created by: yudoufu

fix issue #525 invalid byte sequence error.

I had same problem issue #525. Cause of problem in own case, mix in two encoding, latin1 and UTF-8.

I have mistake mix in a string (maybe latin1 é, not UTF-8). But usualy I use Japanage at UTF-8. so, string fail to recognize on UTF-8 encoding.

Because I fixed invalid string convert to '?'.

Please merge this fix?

sorry for broken my English. ;) Regard.