Emoncms Multigraph fill below curve

Hello
It was cold last night so so I noticed a minor issue with the outside temperature on my Multigraph (otherwise great by the way - well done). The "fill the area below the curve" feature doesn't seem to fill anything less than zero on the vertical axis. The vertical axis auto scales; the line displays a negative value but no fill below zero. Am I doing something wrong?  Apologies if this has already been addressed - I searched but couldn't find any references.