justed tested it and I see what you mean...but this is no bug, this is just the progressbar which 'overlays' the pie chart. you can manually resize the dashboard and see everything or create an advanced XML dashboard and set the hight that fits your needs.
see this pic http://imgur.com/fIvzf where I re-sized the dashboard and you can see the progressbar no longer cuts off the top result.
find the docs about advanced XML here http://docs.splunk.com/Documentation/Splunk/latest/Developer/AdvancedDashboard
I'd still call that a bug... especially in Splunk storm where you're pretty limited on editing capabilities, forcing someone into advanced XML just to fix what should be correctly padded CSS in the first place.
Or you could open up Splunk Storm so we can edit the underlying CSS for our Dashboards/Forms...