plot <data> with fillsteps [above|below] [y=<baseline>]
The fillsteps style is only relvant to 2D plotting.
It is exactly like the style steps except that the area between
the curve and the baseline (default y=0) is filled in the current fill style.
The options above and below fill only the portion to one side of the
baseline. Note that in moving from one data point to the next, both steps
and fillsteps first trace the change in x coordinate and then the change
in y coordinate. See steps (p. ).