Fixed y-axis scale

I have a facetchart to show a heart rate monitor. This chart has a drawline for the upper and lower heart rate. The problem is that the y-axis changes depending on the data the is set on the chart. I would like to force the scale to be 0 (lower) to 140 (high) so both my drawlines are shown, regardless of the current data.

I show 10 samples at most, so the min and max of the data set will be changing over time.

What setting can I use for this?