You can easily do that in JMP. If you wanted to do that using a script, here is an example:
dt = Open( "$SAMPLE_DATA/Big Class.jmp" ); gb1 = dt << Graph Builder(Variables( X( :sex ), X( :age, Position( 1 ) ), Y( :height ) ), Elements( Points( X( 1 ), X( 2 ), Y, Legend( 24 ) ) ) );
If you wanted to use the interactive platform, just drop a variable on the x-axis . Then bring the second variable and hover it on the x-axis, JMP will tell you what will happen depending on where you drop it .
No, unfortunately this is not what I want. Your solution gives me a "nested" x-axis, i.e. in reality two x-axes next to each other. I want them stacked, i.e. both scales span left to right once and each curve is represented by its own axis. Actually, I ask for the very same concept as a secondary y-axis. It just seems not to exist for an x-axis in JMP.
I believe your assessment is correct. I do not think there currently is a way to generate a second X Axis. It is a great idea, so I encourage you to add it to the JMP Wish List.