Ive added secondary y axes before, so you can have a series relating to the
left hand axis and one on the right, but with a common x axis

but is there an easy way to have two x axis, one at the top and one at the
bottom. i don't actually need a 2nd series, but I want my charts to have one
unit across the bottom (dose) and another along the top (exposure index)
just for reference, so that visually I can look at the curve and read off
either dose (at the bottom of the chart), or exposure (at the top)

thanks in advance