What I have understood is that it is a graphical technique used to check if 2 data sources (even if not from the same population) have similar data distribution. In that case the scatter plot will be kind of a straight line. I am not sure when the slope will be exactly 1 (45 degrees). Also is it possible to make a qq plot between 2 continuous variables with different ranges (e.g. the one provided). If so : will the x-axis (age) range from 20 to 70? Or 0 to 70? will the y-axis (salary) range from 5k to 10k? Or 0 to 10k?