Mirror the zero option to the categories plugin.

Due to limitations in our plugin architecture, the categories plugin
duplicates code from Flot's core for adding a dummy point to snap the y
axis to zero.  We can get rid of this duplication in 0.9; for now we'll
just update the duplicate to match the change in core that introduced
the new 'zero' option.

jquery.flot.categories.js

@@ -74,7 +74,8 @@ as "categories" on the axis object, e.g. plot.getAxes().xaxis.categories.
             format.push({ y: true, number: true, required: true });
 
             if (s.bars.show || (s.lines.show && s.lines.fill)) {
-                format.push({ y: true, number: true, required: false, defaultValue: 0 });
+                var autoscale = !!((s.bars.show && s.bars.zero) || (s.lines.show && s.lines.zero));
+                format.push({ y: true, number: true, required: false, defaultValue: 0, autoscale: autoscale });
                 if (s.bars.horizontal) {
                     delete format[format.length - 1].y;
                     format[format.length - 1].x = true;