Ok I took a look at it.
First of all that grey and orange graph looks to be deliberately confusing. It is implied to be the left and right attacks
overlayed on top of each other to make it look like the left edges out almost every year. I think it's the opposite, it's that the exclusively right (grey) and exclusively left (orange) numbers are simply
stacked, not overlapping. Here's another graph from that document. Note the green inflection point that shows where the left is supposedly "outpacing" the right last year:
To imply this data means "the left is more violent now" is pretty much like saying a 450 lb man "consumes less food" than a 78 lb elderly woman because yesterday the elderly woman ate soup, while the 450 lb man happened to not eat anything because he was no longer able to walk to the kitchen on his own.
Right wing extremists are by far more violent than left wing extremists. It's not even close.