Why do contour plots display values at discontinuities using MATLAB 7.2 (R2006a)?

When I create a contour plot of a function which has a discontinuity, MATLAB displays contour lines at the point of the discontinuity as well.

Consider the following example:

[x,y] = meshgrid(-2.05:.1:2.05, -2.05:.1:2.05); 

contour(x,y,atan2(y,x))

Answer 1

[MathWorks Support Team]

This change has been incorporated into the documentation in Release 2011a (R2011a). For previous releases, read below for any additional information:

CONTOUR cannot tell that there is a discontinuity in your data. You need to put in NaN values along the discontinuity to resolve this issue. For example, you might want to put NaN's along the line y=0 for x<0 in this example:

[x,y] = meshgrid(-2.05:.1:2.05,[ -2.05:.1:-0.05,0,0.05:.1:2.05]); 

y(y==0 & x<0) = NaN; % Requires MATLAB 5 

contour(x,y,atan2(y,x))