Added support for hiding and showing individual graph stacks

These changes add support for showing and hiding individual data layers from the GroupStack object, as well as support for showing and hiding the main group's data layer. The new interface functions, `chart.hideStack` and `chart.showStack`, take a group or stack name and show or hide all layers associated with that name. These functions do not automatically re-render the graph. The graph should be rendered (not redrawn) after calling these functions. Redrawing seems to change the chart colors.

I will make a separate pull request with API wiki changes.