one problem is that because you use transparent overlays for the circles, and also colour coding (darker circles are higher values) you get a misleading appeara

one problem is that because you use transparent overlays for the circles, and also colour coding (darker circles are higher values) you get a misleading appearance in heavily sampled areas (which appear darkly coloured just because they have many readings). i'd suggest making the circles opaque (i know it's not going to look as nice, but it will be more useful...)