ive always known this. Unfortunately they are also the most poorly educated and therefore the most easily manipulated by politicians (aka rich republican politicians)
That graph is a bit simplistic to make that conclusion in the OP.
It is no surprise that, various states have different average incomes. And every state has significant numbers of people who despite voting are in the minority Party in their State. You have lots of people voting for both Parties in every single State. And there is a great disparity of wealth between individual's within each State.
I think it is well established by Gallup polls that most of the richest 1% of Americans vote Republican. The 1%ers also are more politically active than the average US citizen.
Maybe a GOP voter is less wealthy on average than a Democrat voter, maybe not. The point is that graph in the OP doesn't show that.