Go to my wiki http://www.tux-tips.de/tux-wiki/doku.php?id=start:dokuwiki:plugins:googlechart here you can find the latest Version of this plugin and also some examples.
<googlechart> some parameters </googlechart>
You can find parameters here Google-Chart API
some special characters are not supported by google-chart, because of URL-encoding.
For Instance the “+” has to be replaced by “%2b”. here an example
<googlechart> cht=tx chl=(a+b)^2 </googlechart> <googlechart> cht=tx chl=(a%2Bb)^2 </googlechart>
here you can find a list of special characters which may also have this problem http://www.w3schools.com/TAGS/ref_urlencode.asp