OK, these charts are getting a bit out there now, so this is basically saying that teams attack the crap out of Jokic and Domas and as C's they're not horrible Vuc level? lol. What does vs. expected mean?
Basically, you can break down each player by their own personal FG% at the rim, and compare it to their FG% at the rim when defender X is guarding them. For instance, one could imagine (fake numbers) that Joel Embiid shoots 68% at the rim, but when he is guarded by Domas, he shoots 77%. That would mean that Embiid shot +9% (77%-68%) at the rim vs. Domas. That number can also be applied to Domas with respect to defending Embiid, and it's the same value (+9%). Which would mean that Domas wasn't very good at defending Embiid at the rim, because Embiid shot 9% better against Domas than he did against "everybody".
Now, if you take the Domas guarding Embiid rim% value, and you add the Domas guarding Ayton rim% value, and you add the Domas guarding Jokic rim% value, ... etc. etc. until you get to every player Domas has defended at the rim, and then you normalize all those values for how many attempts each player had, you get the value we see on the y-axis of the plot above. As you can see, Domas' value is about -3% to -4% - which is GOOD. That means that on average, players shoot worse at the rim when being guarded by Domas relative to being guarded by everybody else.
Now, obviously Domas isn't the best defender at the rim in the league...it look like Giannis takes that one (with fewer total attempts against him). Brook Lopez, with his really good numbers here and large numbers of attempts is probably the most effective player in the league overall at rim defense - and that passes the eye test.
On this plot, the x-axis is just number of shots at the rim defended. Obviously this indicates that teams go pretty hard at both Domas and at Jokic. Of course, Domas also plays big minutes, so that will factor in, and it may actually be that teams don't attack him *that much more* than they attack your average center.
So, all in all, this graph would suggest that Domas is actually not all that bad of a rim defender, especially given his poor reputation. Certainly it suggests he's a good deal better than Vucevic, for whatever that's worth. And also a bit surprisingly, Domas looks to be on par with Embiid. The one caveat that I would have here is that any player whether he be a C or a PG is on this plot if he defended 250 shots at the rim last year. I can't squint through the mass of players on the left side of the plot, but I imagine that many of the players above the "average" line are guards, whose rim-protection abilities aren't really expected to be that good. It would be nice to see this plot where only players, say, 6'9"+ are included, and to have the average rim% value recalculated for only shots contested by a 6'9"+ player to see if Domas still ends up above average. But at the very least, if your defensive numbers look like Embiid's, you can't really be a gaping wound on defense.