You can use a new feature of 6.0 to mask out any shaped region you like. Drawing on some high school trigonometry I was able to come up with the following LabTalkTM script that works on an existing Contour Plot:
%M=%H;
win -a m248x248;
matrix -pg DIM ncol nrow;
matrix -pg X XMin XMax;
matrix -pg Y YMin YMax;
win -a %M;
win -t data origin;
%N=%H;
// Do the math for an Ellipse
a=(XMax-XMin)/2;
b=(YMax-YMin)/2;
px0=XMin+a;
py0=YMin+b;
stepsize=int(sqrt(ncol^2+nrow^2));
for(px=XMin,row=1;px<=XMax;px+=((XMax-XMin)/stepsize),row+=1) {
py=sqrt(b^2*(a^2-(px-px0)^2)/a^2)+py0;
%H_A[row]=px;
%H_B[row]=py;
};
row--;
for(px=XMax;px>=XMin;px-=((XMax-XMin)/stepsize),row+=1) {
py=-sqrt(b^2*(a^2-(px-px0)^2)/a^2)+py0;
%H_A[row]=px;
%H_B[row]=py;
}
// and add the data to the contour
win -a %M;
layer -i %N_B;
label -r legend;
(Warning! no checks are performed to verify that you have a contour plot as the active window. Also, if the contour contains other datasets, you must substitute the matrix name for %C.)
Once the script completes, you should open Plot Details for the added dataset and check the Fill Area Under Curve box and select Exclusive broken by missing values from the drop-down list. Then select the Pattern Tab and set the Fill Color to White (or whatever).
You can change the math to create any region you like or just use the Draw Data tool to create any arbitrary shape.
Users of previous versions must create four datasets which are grouped in pairs (top mask and bottom mask) and set with fill color and drop lines.
