It's not a completely unreasonable thing to say, at most it's phrased a bit sloppily. As far as I remember, when you set up the graph there's nothing guiding th

It's not a completely unreasonable thing to say, at most it's phrased a bit sloppily. As far as I remember, when you set up the graph there's nothing guiding the user to use limits for the graph that result in a proper 1:1 aspect ratio. Instead if you e.g. want a graph from -3:3 on the X axis, you probably also set up the Y axis as -3:3 too. Since the real screen isn't 1:1, this will result in the graph being squashed down. And except for the special case of lines that are parallel with the screen edges, a stretched or squashed aspect ratio would indeed distort the angles such that you can't tell which angles are perpendicular and which ones aren't.