Radar Chart

Ok, here goes - I am using Radar charts on Excel quite a bit and generally there are 4 or more series of data to compare. This is done with no problems but when I get down to fewer series of data the radar chart is unable to format into a relevant shape.

For example with 4 series of data I get a square, with 8 an octagon, 10 a decagon etc.

When I get down to 2 or 1 series of data I would like the radar chart to format into a circle, with 100% being the maximum value and the 80% (for example) the achieved value shown as a circle within a circle.

I have attached a spreadsheet to help demonstrate. The lovely drawing is all my creation

Is this possible and if so how do I go about getting the radar chart to act as I require?

Many thanks

Dan