Option to draw grid lines in SpiderWebChart

[jochenberger]

I'd like to be able to draw grid lines in a spider web chart. There's been some discussion and even a solution from @jfree: https://www.jfree.org/forum/viewtopic.php?f=3&t=115667#p174682

[jochenberger]

I'd also appreciate options to draw the lines as circles instead of polygons and to show the numeric values that the lines represent.
And maybe gridLineCount should be replaced by gridLineInterval or something similar.