Can you post a scanned example of what you are trying to produce - I am not sure why you would want the hyetograph "upside down above the hydrograph".
Tricks you might consider to get the plot to appear the way you want it:
1. Over-write the default y-axis scales, so as to shift the minimum and maximum values for each axis, so that the graphs appear "stacked" on top of each other, instead of both plotting from the same base-line.
2. Plot "Values in reverse order" to plot one graph "upside down".
See attached sample - is this the sort of thing you want?