There are a couple of issues, I think.
A drop-down list is single-selection. So, you could not use it to filter the one graph by multiple choices.
A single control interacts with all associated objects in the same way. You cannot specify to filter Graph1 one way and Graph2 a different way through the same interaction.
I would suggest using the dropdown list to filter all of your graphs except the one, and adding a list control (which supports multiple selections) to filter the other graph.
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