You don't have to set zero as the baseline in a line chart.
As you probably noticed,
ggplot automatically chooses the limits of the vertical and horizontal axes. If you look carefully at the vertical axis, you will see that – contrary to a bar chart – it doesn't start at zero. Is this a mistake?
No, it isn't. Whether the vertical axis should start at zero depends on the chart type. Since a bar chart encodes values in bar length, trimming its axis can be misleading. But a line chart encodes data based on the vertical position of a point, which doesn't require zero as a baseline. It's the relative distances between points that matter, and these won't change with a changing baseline.
However, this doesn't mean you should never use zero as a line chart's baseline. Consider which baseline will be more meaningful and logical. If your numerical values start always at 50, there's no point in starting your chart at zero – it makes for a lot of unused space under the line. Also, the line will be needlessly flattened, which will make trends harder to read. But if examining your data makes you feel that zero is a good baseline, then use it.