Autist, did you move up stakes at around 50,000 hands? Did you bump up the number of tables you play? That is a huge change in your non-SD winnings at that point of the graph. You played almost 50,000 hands break even in non-sd pots, which is a very good performance, and then started tailing off. It seems obvious that something changed.
If you did not move up to tougher stakes, or add more tables, then you should try to figure out what you were doing differently earlier and go back to it. If you made either of these changes at around hand 50,000, it is probably the explanation; all of the leaks I have described below are magnified by moving up or playing too many tables.
The steadily declining non-sd line is an indicator of "weak-tight" play. Typically, it is characterized by 4 leaks, all of which you will need to address:
1. Putting too much money in when behind, usually by calling a villain's bet. Use your tracking software to filter for hands that you lost that DID NOT go to showdown. check to see if you could have/should have folded earlier in the hand. If you are floating a lot of flops only to fold to a turn bet, that is going to start to add up. You will just have to go through the hands and determine whether a raise or a fold or getting to showdown was a better play in each hand/type of situation.
2. Not firing at good flops. It is easy to hit the "check," button when you are in a blind and the flop misses you (especially when mass multi-tabling). But you have to adopt the mentality that has you examining the flop to see if it probably hit your opponents, and then you fire away with any pair or draw at all when you think the flop missed them. These "orphan pots," go a long way toward reducing your losses from the blinds.
3. Letting your opponent get to showdown second best too often. This is really a failure to extract maximum value. The typical pot control line--bet flop, check turn, bet river ensures that a lot of the good hands you have become showdown winnings, rather than non-showdown winnings, as you have made showdown reasonably priced for the villain. This is a fine line sometimes. But the line bet flop, bet turn usually induces more folds because it is so much stronger than the pot control line. If you have a steadily declining non-sd line, it probably indicates that your default betting line is bet flop, check turn with weak hands you play in the blinds. Similarly, checking behind on the river with the best hand is a frequent failure to extract maximum value, and it also moves into the showdown winnings columns a fair number of pots that the villain would have folded to a river bet and would have become non-showdown winnings that way.
IMPORTANT NOTE:We don't care about whether a win is a SD or a non-SD win, except for the fact that if we are in position and checking behind too much (failing to extract max value), one of the ways we can find this out is by examining the slope of our non-SD line. Like I said in the OP, we don't care about our stats being pretty, or "correct," we only care about them because they tell us what is wrong with our play.
Because your problem is mainly in the blinds, you should take a look at the hands where you fire at a small pot in the blinds and then check the turn. What was the result of the hand? maybe a second bet could have taken it down? Or maybe you won the hand at showdown? anyway, consider firing more turns where it seems unlikely that the villain has anything. Don't automatically give up on a pot just because the villain called your flop bet. Don't automatically go to a pot control line just because the villain hangs around.
4. Not defending enough. this is an easy concept to grasp, but so, so hard to implement. If you are in the blinds and the villain on the button has an attempt to steal % in the 30s, you KNOW he is often raising you with almost trash. So why are you folding your decent hands? Sure, his position makes up for a lot, but you have three responses to his positional advantage. 1. Seize the initiative by 3 betting (your 3 bet stats looked fine, imo) 2. call with hands that are, on average, a little better than the hands he is raising with and 3. you have superior hand reading skills, and you will play back at him aggressively on flops that missed his range, or hands that look like they hit yours, or flops where he just can't take a lot of pressure. Your defense stats look pretty appropriate, so you ought to go through those hands and see whether you are winning or losing in those hands. It seems pretty likely that you are playing too much "fit or fold," when you defend.
Your graphs actually look pretty good. Most TAg players lose money in non-SD pots (including the blinds) and win in non-SD pots when the blinds are excluded. So your graph is pretty normal. But if you focus on identifying any of these leaks you might have in the blinds, you ought to be able to reduce the downward slope.
Good luck.