Robert Kosara has an interesting piece about the “banking to 45-degrees” phenomena in line-charts and its origin in a 1988 paper.
What people often miss, however, is what task Cleveland and the McGills were after. The paper is very specifically about the comparison between the slopes of two lines, and the slope is the average between those two lines. So if the goal is to be able to compare the rates of change between lines, the 45º average slope makes sense as a rule. It may be a good idea in other circumstances as well, but this particular study does not offer any information to support that.
Interestingly, at last year’s InfoVis some researchers replicated the work and found it not entirely accurate, but rather the 45-degree result was a result of boundary conditions.
