To make an y-axis start at zero is important to not distort the magnitude of a trend. E.g. a change in fans from 1000 to 1010 looks huge, if the y-range goes from 990 to 1020. However, it looks rather small if the y-range goes from 0 to 1020.
Currently I am using Area charts since their y-axis starts at zero by default. But I would be happier, if I could use plain line charts.