Pressure/Height vs. Time (high-level plot interface)
Data must be in pressure coordinates!
gsn_csm_pres_hgt is the plot interface that creates time vs. height plots.
Note, the data must be on pressure levels to use this interface.
ShadeLtGtContour is the function that does the shading.
There are numerous contour effects to choose from.
tmYRTickSpacingF controls the spacing of the height labels.