I would prefer this be passed in via the bundle, but it continues to fail
saying "Unknown Provider". I have chosen to require them into the module
the old-fashioned way, then allow an injectible dependency to override.
Extends PlotController by adding three new scoped methods:
exportPDF, exportPNG, exportJPG.
All three methods use basically the same steps. The HTML node
of the plot is passed through html2canvas which generates a canvas.
From the canvas we export a blob, PNG, or JPG then save the file.
