Show circles of estimate telemetry radio range

Per Tridge's wiki entry:
If the 'rssi' and 'noise' levels meet on the graph then you will lose the link. To determine what your range would be, a rough rule of thumb is to subtract the 'rssi' and 'noise' numbers, then divide by 2. That tells you your "fade margin" in decibels. For each 6dB of fade margin your range doubles. So if you have 18dB of fade margin, then you will be able to do roughly 8x whatever range you were at when you measured the margin.