Search the internet first.
Oh thank you so much! I truly did not think to try to search the internet before coming here at all....
(I was being sarcastic if you didn't pick it up)
After NOT being able to find anything using keywords "Disodium Tartrate Dihydrate", "Disodium Tartrate", "TGA", "decomposition", and "+ heat", I found Figure 4 of this article a few hours after posting here.
G. Panzarasa, “Rediscovering Pyrotartaric Acid: A Chemical Interpretation of the Volatile Salt of Tartar,” Bull. Hist. Chem., 2015, 40, 1-8.
Such a helpful answer in this forum. Not.