One of the things we noticed when mapping Noda Bi-Yehuda is that the map for Vol. I looks different from the map of Vol. II, and that his sphere of correspondence seems to shrink over time. Take a look at what I wrote in the post about this.
We now have another way to visualize this: distinct heatmaps for the two volumes. You really see how Vol II covers a far more compact area, especially relative to the overall number of responsa. Take a look: