Thanks for your reply! I am not sure I follow your train of thoughts. Could you write some dummy code to illustrate what you mean?
If I understood you correctly, it looks like you suggest storing a constant dataframe as a dash.dependencies.State in the callback for fetching data, so that i can always compare the current data to the constant one. But I may have misunderstood you. If so I apologize 
I want to compare the output of the current callback to the output of the previous callback - always “one callback earlier in time”.