That first stacked bar chart appears to have "left" and "right" mixed up, if the analysis just below it is correct. Not confidence-inspiring, and hasn't been corrected in 18 months since the post was published...
The analysis seems to disagree with the data shown in the graphs about the labels of "left" and "right". The author seems to be confusing left/right? It's not just the first stacked bar, but also the third (avg %bias)
I think what you’re seeing is a confusion coming from ground news own terms. They call something a “left wing blind spot” when it’s a story that does not appear in left-wing sources, meaning that it only appears in right wing sources. The blog copies that terminology here, but it can lead to some confusion.