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).

Yes Numbers does now supports two axes. Steps are:

1. Select the chart, change the type to 2D 2-Axis
2. Make sure you have more than one data series selected.
3. The Series tab for each data series will allow you select which axis to use for each series and the type: columns, lines or areas.
4. The chart Axis tab contains (Y1) and (Y2) tabs which you can customise.

If you are familiar with customising graphs in Numbers - it isn't complicated.