How can I use a secondary axis in Numbers?
If you want a line graph with 2 y-axes. You to insert->Chart-> 2-Axis. Put in your data. One of the sets will be a line the others a bar. The way to fix that is: Select the one that is a bar then to go to the Inspector-> Chart-> Series. It will say series type-> select that and change it from bar to line. Right below that it says plot on: There you can determine if you want that line to be plotted on y(1) or y(2).