I had a few issues because I tried to modify the low value triangle to be of the right color instead of white.
I dont know if anybody plans to modify this somewhere, but this is the way i solved the problem :
1) I changed the white triangle color to the corresponding low value color
2) I changed the color of the rectangles to be defined by the value of L(i) + L(i+1) / 2 instead of just L(i), so that the lower triangle does not have the same color as the lowest rectangle.
3) I then change the colormap used in the plot because in contourf the colors used to fill are the colors of the lower limit, (the color of L(i), and not L(i) + L(i+1)), so I take for example the jet map, and chop a part of the top and place it at the bottom.
19 Feb 2014
Compute the cumulative sum of matrices, specifying the treatment of NaNs.
Keith - I just submitted a fix for that case. The new version should appear online in the morning. It happens because the result was essentially zero for some elements, but floating point arithmetic errors can drive a value less than zero, then a sqrt is taken.