There's Polar charts available in the Mendix Chart module. I haven't used that chart, but it might suit your needs.
You can preview the polar chart on this page if you scroll down a bit.
There is also a spider chart widget in the appstore but its from 2012 so most likely its outdated.
You can also take a look at a Radar chart from the chart js library.
I’ve seen a spider chart in a mx6 app but can’t remember the exact widget that was used. I think the Polar chart (as Larn said) is the most similar chart that is available in mx8.
If you like a widget that isnt compatible with mx8, you can try the trick I mentioned in this question: