With two more solo homers on Sunday, that means 16 of his team-leading 18 bombs this season have come with the bases empty. Combine that with the fact that he has a .482 OPS with runners on base and a 1.226 OPS with the bases empty, and factor in that he only has 32 hits all season that didn't leave the yard, and you get 18 home runs with just 31 RBI.
It's a pretty small sample size, and we do buy into the idea that RBIs are somewhat circumstantial (though we will never fully discard the idea that some hitters are better in the "clutch" than others, i.e. able to put aside external pressures to produce during the task at hand).
In any event, at the very-near midpoint of the season, if you simply doubled Plouffe's totals, he would finish the year with 36 homers and 62 RBI. If that seems like an odd stat line, well, it is. How odd? Historic.
Check this list from Baseball Reference from 2011: Plouffe, if we simply double his numbers, would finish the year with the fewest RBI in history of any player to hit more than 30 homers. Also: Barry Bonds is the record-holder in terms of lowest ratio of RBI to HRs of anyone with at least 20 homers in a season, getting "only" 137 RBI to go with his record-setting 73 HRs in 2001. That works out to 1.88 RBI per homer. Plouffe, as of right now, has 1.72 RBI per home run, easily on pace to break that record.
Our best guess, of course, is that Plouffe will start hitting some of those three-run homers that Josh Willingham and Drew Butera cracked Sunday, and that he will also start getting some non-homer RBIs (it's absolutely stunning, for instance, that Plouffe has more RBI on solo homers, 16, than on every other type of hit, 15). If he keeps cranking at this rate, it wouldn't be crazy to think he'll end the year with 30-35 homers and 80-85 RBI. For now, though, he is on pace to make history.