Define the labels for the axes. The AXIS1 statement defines the axis properties for the bar response (left) axis. The AXIS2 statement defines the properties for the plot response (right) axis. The AXIS3 statement is used to suppress the default label on the midpoint axis.

axis1 label=(j=c "Revenue" j=c "(billions)") minor=none; /* left */
axis2 label=(j=c "Power" j=c "Generated" j=c "(GWh)") minor=none; /* right */
axis3 label=none; /* bottom */