There is an axis option for each series (Left or Right). The right axis won't show up unless there is a series pointing to it. So, just add two series, and have one show the Temp in C and the other the Temp in F. One uses the left axis; the other uses the right axis. The series might overlap depending on the scale options, but turning off the line and shrinking the point size to 1 can hide what is not desired to display.