Tammy LaBiche-Henley thinks this is interesting:

The rule we’ve illustrated here is that bar charts must have a zero baseline. Note that this rule does not apply to line graphs. With line graphs, since the focus is on the relative position in space (rather than the length from the baseline or axis), you can get away with a nonzero baseline. Still, you should approach with caution—make it clear to your audience that you are using a nonzero baseline and take context into account so you don’t overzoom and make minor changes or differences appear significant.