I would not count it as a visit in my records. I wouldn't even count it if it were rain-shortened. Sorry to say that but it should not count.
1 update is that the game was September 21, not the 19th.
It's up to you really. To me "chasing" is seeing a game in every ballpark, so while I would be very disappointed in not counting it and having to go back, those are just my rules. For instance I was in Cleveland for a cavs/celtics game and went to progressive field (Cleveland Indians) but i have not marked it off my list because there was no game being played there that day. If your goal of "chasing" is just to visit every ballpark and see what it has to offer, buy a hotdog, then you've done that and it would count.
I would consider this as an official ballpark since the purpose of your visit was to see a game. Though I am in obvious disagreement with some of the other members that have commented, my feeling is that if I enter (key word enter) the stadium on a game day with the intent of seeing a game, I count it. There was only one stadium I have been to in which the game was rained out, and that was Portland Oregon PGE Park. I was still able to walk around the stadium, take pictures, buy a program and home team cap, and enjoy the stadium, even though it was soaked. Since mother nature doesn't always cooperate, I don't think we should be penalized in not including a stadium that we have been in. I have mixed feelings on stadium tours. Any thoughts on stadium tours, should they count, yes or no?
I don't count my tour of the Rangers Stadium in Arlington. As far ad I a, concerned, it needs to be an official game.
My personal criterion has always been to count a ballpark visit only if an "official" game is played (4 1/2 or 5 innings as appropriate). I've never seen a rain-shortened game, official or unofficial, but have been rained out several times, a few times after sitting in the ballpark for hours hoping a game would be played. In those cases I have returned at a later date to complete my "visit." However, we chasers have no official rule book, so you can set whatever criteria make you happy and comfortable in your own ballpark chasing, and ultimately you answer only to yourself.
I feel if I make the trip and buy a ticket and enter the stadium, it counts. I made a road trip to Pulaski, Virginia last year. I watched batting practice from on top of the hill, and then there was a huge 30-minute storm. When the gates opened, the rain was finished and I watched the players warm up for an hour. Even bought a scorecard. But the field was too wet so they couldn't play. I personally count this on my list. However, the next day, I stopped off in Princeton, WV to see the P-Rays play. I walked around the stadium, but never went inside because it started to storm (game eventually got rained out). I don't count Princeton in my lifetime list. Ultimately, it's really up to you.