Enable optional transparency in PlotKit

gistfile1.diff

[artemgr@pcbsd /tmp/plotkit-patches]$ git diff
diff --git a/PlotKit/SweetCanvas.js b/PlotKit/SweetCanvas.js
index dcfdfc5..39cbf2c 100644
--- a/PlotKit/SweetCanvas.js
+++ b/PlotKit/SweetCanvas.js
@@ -195,6 +195,10 @@ PlotKit.SweetCanvasRenderer.prototype._renderLineChart = function() {

         // faux shadow for firefox
         if (this.options.shouldFill) {
+           // Transparent fill.
+           // See "http://groups.google.com/group/plotkit/browse_frm/thread/8e86898adce15bd7".
+           if (this.options.globalAlpha) context.globalAlpha = this.options.globalAlpha;
+
             context.save();
             if (this.isIE) {
                 context.fillStyle = "#cccccc";