Sets or returns the size of the center hole of a doughnut chart as a percentage of the doughnut size.
This defaults to 50 when creating a new doughnut series.
If this is set to a value less than 10, it will be set to 10.
If this is set to a value greater than 90 it will be set to 90.
This property is linked for doughnut series in the same group. See SettingsPieDoughnut for more information on how linking works.
For series that are not doughnut series, setting this will have no effect and it will always return 0.