Sure, we could. However , the fundamental problem is that the notion of "circle" with "radius" really assume a 1-1 aspect ratio. Picking one dimension automatically based on some criteria is still picking one dimension, and does not really address the problem. And now it might change unexpectedly if the plot is resized.  If we are going to do anything I would say making the dimension configurable: "x", "y", "min", "max" and then optionally issuing warnings if the aspect ratio is not 1-1