I see, you want to count the multi selection values in the list based on single selection value.
I don't think that this is possible. The XAxis value is treat as a string and has no capacity of counting inside strings.
I'm thinking of a helper list here. A list that would hold the individual selection adn the counting. This can be accomplished with some coding.
You desired chart screenshot comes from a different list not from the list you want to use, right?