I think you've found a bug.... you win a cookie

It seems that the working day colouring (and the greyout colouring probably) go from 0 to +Inf and so if the graph is negitive then they dont colour the bottom half.
If I remember right (from a long time back) this was because a bug in RRDTool under Windows meant that if you specified to colour from -Inf to +Inf you in fact got nothing -- -Inf was treated as +Inf. Also, refering to -ve number when the line was only +ve messed up the axis (you had part of the graph -ve even though the line was always +ve).
This may be fixed now, so I might re-try it. Not sure about RRD 1.x compatibility though.