Doesn't clabel require C as an input argument, where C is the output of a contour command? If so, I don't have this. I have (x,y) pairs that define the level curves. For example, I have a 100x2 matrix, each row being a point along the curve. I plot the level curve using the plot command, i.e., plot(x(:,1),x(:,2)). Plot returns a handle, but not a C matrix.
Am I missing something?