on point 2 : I'd like to point out that the 2 axes of Y do not have to be the same thing or share the same unit. Hence their values and zero points may not necessarily align. That is to say, there are genuine charts that look like the 2nd on the left. The requirement is that a point need only satisfy (x,y1) and (x, y2) together but that doesn't mean the point has to satisfy (y1,y2) as well. You can find lots of charts like this in experimental research (especially engineering) where an author may condense multiple experiments onto a single graph for comparison, or compare between more than 2 discreet variables.