At the moment, you can only use a Graph widget to display values > than 0%. For example, if you have a Graph widget displaying Tasks graphed by who they're assigned to, then you will only see People who have assigned Tasks. There is no current way to display People in a Graph Widget who have no Tasks or Tasks = 0%. If we were to include 0% in the Graph widget logic then you could see People who are yet to be assigned Tasks.
Please add your support to this feature request if you'd like to see 0% displayed on the Graph widget.