Heatmaps for Geiger counts may be misleading (at least at this stage), because: Geiger Counters are highly localized and too few compared to temperature sensors. Ionization counts do not diffuse the way temperatures diffuse.
A radiatiom heat map is shown in a way at Safecast.org, which uses moving detectors on single trips. Turn on layers, then choose points (the top dates show the most data). Radiation is shown as color coded tracks as the meters move along roads and trails. It shows that radiation readings mainly follow elevation (no surprise). Of course, readings near Uranium mines and nuclear fallout areas show higher values. Fixed detectors showing radiation over time like on this site don't really lend themselves to heatmaps.