CanvasAxisTextRenderer does not convert & to &
When using the CanvasAxisLabelRenderer & characters are not converted back to & before being displayed. The data should be stored as & so that the page validates but then should be converted to & when being displayed.
This short jQuery snipit can do the conversion for you easily.
I was able to get it to work by changing two lines in jqplot.canvasTextRenderer.js
110 this.text = $('<div />').html(t).text();
415 ctx.fillText($('<div />').html(str).text(), x, y);
In my short tests this doesn't break any compatibility with existing features.