unicode for wxagg and gtkagg backends

Hello,

Since I have problems with unicode conversion when using wxagg and gtkagg
backends I would like to ask which files in matplotlib sources are
responsible for this?
To understand the mechanism, if in the figure I get empty squares
instead of special characters where is the problem: in the phase of
processing a text by antigrain or earlier?

Thanks in advance for any hints,
regards,
SK