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.