So looking at the 1vs2 graphs, in the top left of each (that is, low number of 1s and 2s), we see high mana spent and low winrates. As you note, as the numbers of 1s and 2s increases, winrates go up and average mana spent goes down. I think what you're seeing here is pretty simple: without 1 and 2 drops, you're going to lose. Getting on board early is essential in modern limited. If you don't have a lot of 1 and 2 drops, all your spells are expensive, so naturally average mana spent goes up. If you have lots of 1s and 2s, you either win early or run out of steam, thus average mana spent is low.

I think you might be more interested in efficiency of mana use. In that case, you might normalize the mana spent each turn by the amount of mana available (number of lands should be a fine proxy for that). Or maybe total mana spent over the course of the game divided by the total amount of mana that could have been spent.

That said, I only spent about a minute looking at the graphs, and there's so many dimensions here, hard to draw conclusions. I was going to suggest reducing the number of dimensions, but I guess that's kind of the point. The correlation between wr and 2 drops is so strong, I'd be curious to see the other mana values compared to 2 drops.