Ring plots with no holes should behave like ring series with a pie chart in the
middle. However, setting the hole size to zero causes a single series to be
displayed. 

1) open the attached file
2) click on the ring plot with one of the secondary mouse buttons to get the
context menu
3) select properties
4) select the RingPlot element
5) reduce the center size to zero
==> up to 5% things behave correctly, however at 0% only one plot becomes
visible and the overall graph becomes a pie chart.