Allow overriding of Radar Chart min/max values for tick calculation

[zmeggyesi]

In cases where the possible minimum/maximum values are known in advance, it should be possible to use these to determine the starting/ending ticks of Radar Charts. This would allow stable and comparable charts across disparate data sets.

Currently, the tick spacing and the supporting titles are calculated by taking the minimum and maximum data entry values, which is fine for the general case, but makes it hard to compare datasets against absolute margins, as the ticks change position according to the entries.