Question:
Firebug console throws "a.jqplot.CanvasFontRenderer is not a constructor" from within "jqplot.canvasaxislabelrenderer.min.js" plugin. 

Answer:

Please take a look at those posts:
http://phpchart.org/tag/plugin/

It explains the renderer and non-renderer plugins. CanvasTextRenderer is a non-renderer plugin, so must use add_plugins() method.