If you don't specify an "order={min} to {max} by {something}" in your axis statements, then the plot and plot2 axes will each auto-scale based on the data that's being plotted against that axis.
One alternative to plotting the 2 lines via plot and plot2 is to plot them both with the plot statement (using the 'overlay' option). You will only get an axis on the left side, and you are guaranteed that axis is auto-scaled the same for both lines (ie, because it *is* the same axis 🙂