The PHPlot Class is very old (2003) and it does not support utf8 chars by default.
You can fix strange characters in the Graph Title changing a singe line on the file source.
1) Open the /admin/includes/classes/phplot.php
2) Go to line : 609
3) Replace "$this->title_txt = $title;" by "$this->title_txt = utf8_decode($title);"