I don't think you can do it out of the box, but maybe by using two variables for the counting?!
Assign the value 1 to the first one and -1 to the second.
Assign a parameter to your buttons.
Then create a new calculated measure that checks the parameters value and either use the first variable (1) or the second (-1).
Then use this measure in your graph and do the ranking.
I have not tested it, but it could work 🙂
//Fredrik