I do need that same variable in the dropdown list (which has 3 choices) to control all the graphs in the same canvas (one choice by one), but one of the graphs needs 2 choices fixed of the 3 choices from the dropdown list. Can you help? Thanks!
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.