I wonder what the data would look like in the theoretical scenario where Disc was "balanced" so that Holy felt like a genuine playstyle choice to the playbase rather than a no-brainer (perceived or otherwise).
For a glimpse into that, you can look back to a brief period when Holy was balanced with the other specs but Disc was slightly weaker and had limited absorbs:
Check out the beginning of that graph, up to about patch 4.2.
You can also look at the Dragon Soul (4.3) period in 25s, where the two specs were legitimately balanced.
Unfortunately, neither of those includes monks. There's a very brief period in 5.0.5 where Holy outstrips Disc again, but since Holy still wasn't balanced with the other specs and since the hotfix for Disc came out pretty quickly (and everyone knew it had to be coming), you don't see as large an effect as you do in 4.0.6-4.1 and in 4.3.