When exporting graphs to any format, if you are using a custom font that is not installed on the user's computer (even if you are using the CSS @font declaration to include it for your site), the font will not be used on the exported chart.

Example:
http://jsfiddle.net/n2rcogxa/

You should be able to solve the issue by embedding the base64-encoded font in the SVG document prior to converting from SVG to whatever the desired format is (PNG, JPG, PDF, etc).

Example:
http://jsfiddle.net/moyf3zvb/2/

Note you must actually embed the font. Including it as a @font declaration in the SVG document does not work.