Then I'm confused?! I couldn't work out why the grey line was thin compared to the blue line. You've not used soft edge etc and the border has no line. I noticed the plot area has a fill set to 'Picture or texture fill' and setting it to no fill etc removes the grey circle.
If it's not an image...
Hui, loving the use of a circle pic as the back ground to add a different visualisation of a doughnut chart. Drove me nuts intially trying to work out how it was done 'til in struck me. Will be employing this method going forward :)