[jmuyskens]

Bar charts use length to show amount while line charts use slope to show change. Therefore, it is misleading to cut off the y-axis for a bar chart, but not for a line chart so long as slopes are transformed proportionally. For line charts, it is generally best to constrain the y-axis such that change (keeping in mind the precision of the data) can be discerned. Of course, if zero happens to be meaningful, it could be illuminating to include and annotate it, just as you would any other relevant point of reference. However, in the case of the temperature chart, zero degrees fahrenheit is arbitrary. Ask yourself, why not zero degrees kelvin?

Agenda will show through. Hopefully, in data viz, your agenda is scientific/statistical accuracy.