I have four lines that each define a horizontal zone. When I change each to
an Area Chart, it appears that the color is only painted above each line and
the color extends from the line all the way to the top of the chart. But
above the top line I'd like this to be uncolored. I want the top two areas
are filled in BELOW each line to a horiz. mid-point on the chart and the
bottom two areas are filled in ABOVE these lines to a horiz. mid-point. In
effect I want the chart to depict from top to bottom:

Empty space
Danger zone
Safe zone
Safe zone
Danger zone
Empty space

Help!!