To your first point, I agree that the extremists on the right have gotten a lot more extreme. I don't post much about neo-Nazis or white nationalists because my guess is that we all already agree about them. My concern is that the extreme left is splintering liberals. My guess is that most everyone posting on this thread pretty much share the same values. Maybe slight differences between us, but for the most part, we value the same things. What worries me is that the extreme left seems hell bent on dividing us.
I think you might be surprised by how many people would disagree with us on white supremacism and neo-Nazism, especially once you got down to specific and smaller but still harmful behaviors such as displaying the Confederate flag.
I think extremists of all sorts tend to cause unnecessary division and strife. Trump has caused a rift among some members of his own party due to his polarizing politics, and Cenk Uygur remains controversial among the left largely due to being a "tankie." It's not a phenomenon exclusive to the left, much less to people who call out right-wing extremism.
Also, sometimes there are good reasons to cause division or expose flaws in certain political positions, which could lead to division and controversy. If you call out a liberal for being too cozy with Saudi Arabia's theocratic tyranny, you are almost guaranteed to be accused of "racism" or "Islamophobia" by some extremists, but that sort of division is both unavoidable in any movement and useful for exposing the fringe, unreasonable elements among its ranks.
I'm not sure I understand your 2nd paragraph, can you restate it?
The diagram portrays a shift in the "fellow liberal's" position on the spectrum but portrays the far end of the right wing as fixed. This is incorrect: the polarizarion and growing divide among the left and right is bipartisan and has many more reasons than just "progressives have become too extreme."
A solid demonstration of this is to compare what counts as "leftist" in the U.S. to, say, Sweden or Australia. American "leftist" politics is largely more conservative than it is in multiple other parts of the developed world, which means American "centrist" politics also leans farther to the right.