You can change axis configurations in "Axis Setting" dialog. You can acesss it from "Y1/Y2 Assignment" Y-Axis menu.
You can change the following configurations.
Assign to Y2: If you check it, the column is assigned to Y2 axis (on the right hand side of the chart).
Show Axis Names: If you check it, it shows which axis (either "(Y1)" or "(Y2)") is assigned for each column in the legend.
This menu currently shows up in Y-Axis menu only when you have multiple columns at Y-Axis.