Robert Kosara has a great summary of the "banking to 45 degrees" practice first proposed by Bill Cleveland (link). Roughly speaking, the idea is that the slope of a line chart should be close to 45 degrees for the best perception. It's not a rule that you see much on Junk Charts because it's one of those rules about which I don't hold a strong opinion.
Here are the examples given by Kosara:
The same data is presented three ways. The slope is a reflection of the scales used on the two axes.
Well, I lied when I said I didn't care. Look at this particular chart below:
Some of you may recognize this style... I'm imitating Google Analytics charts. Several of the other Web charting tools also seem to come up with gems like this. Pretty much every chart you see in the Google Analytics interface looks like a flat line. The chart above looks like nothing more than noisy data from week to week.
But then look at the scale! The leftmost part of the line is a rise over two weeks. The actual rise was 50% or 300,000, i.e. an earth-shattering change.
If you use Google Analytics, you are better off downloading the data to Excel and drawing your own charts.