The top graph on page 6 looks a bit dodgy to me. First of all it's for a jet (while all/most of the discussion above is for a prop) and secondly the effect of altitude on the power required curve is to move it up and right along a tangent drawn from the origin; the power required curve in this diagram has hardly moved up at all, certainly not along a tangent. It's a problem with so many of these graphs in textbooks, they are not taken from hard data, just 'illustrative'.