Examples of line graphs used in a visual perception experiment

Reproduced from Cleveland, W.S. & Mcgill, R. 1984. Graphical Perception - Theory, Experimentation, and Application to the Development of Graphical Methods. Journal of the American Statistical Association, 79, (387) 531-554.
The line graph above shows the differences in curves from Cleveland and McGill (1984). To the left, two curves are displayed, and volunteers were asked to compare the vertical difference. To the right, the actual difference between the two curves is displayed. The experiment showed that it was difficult and inaccurate to estimate the vertical differences between two curves on the same graph. Human perception interprets the difference in terms of the minimum vertical distance in different regions of the graphs, thus seeing that the curves are getting closer together, going from left to right when it is not actually the case.