I assume you want to use PHP for the graphs to ensure that everyone who views the site can see them? Why not use a JS library such as Plotr to create the spangly 3D version, but fall back on CSS for those users without the correct browser support;
http://speckyboy.com/2009/02/04/16-usable-css-graph-and-bar-chart-tutorials-and-techniques/
EDIT
If the graph is being sent via email then you can t use JS, but there s no reason why you can t use CSS. Get PHP to output a list, or a series of inputs (whatever best fit s your purpose), send the email in an HTML format, and use CSS to style the HMTL using one of the techniques illustrated in the link above.
Also, using PHP to generate static images will present other issues, most notably getting through user s spam filters and your own bandwidth, not to mention how much of a waste of CPU cycles it is to process these images when a bit of HTML / CSS trickery would achieve the same effect.