From support e-mail:
Q: Will DPLOT compute the area and or acreage within a closed plotted polygon?
A: Yes. If you use the $AREA shortcut in any text entry (title lines, axis labels, legend, etc.) DPlot will report the area enclosed by your polygon. If the points are arranged in a clockwise direction the value will be positive; if counterclockwise the value will be negative. You can force a positive answer by using the absolute value operation, as in "Area=$=(ABS($AREA))". The initial "$=" tells DPlot an equation follows, which must be enclosed by parentheses.
If you have more than 1 curve then you'll need to use $AREA(n), where n is the 1-based index of the curve.
For most applications (and in particular since you asked about acreage) the value replacing $AREA is perfect. If you're trying to find the area enclosed by what should be a smooth curve, though, you might need to generate more points, if possible. The circle below was generated with X=f(t), Y=g(t) with X=2*cos(t), Y=2*sin(t) with the spacing in t set to 1 degree. The difference between the calculated area and the well-known area of a circle is due to the spacing between points.
In case anybody is wondering, the plain text for that text note is:
R =2
Area =$AREA
{\sp}r{\u2} =$=(PI*4)
with the tab stop set to 5 characters.
For more information see How do I find the area under a curve? in the online manual.
