Multiple graphs on one plot

Hi All,

I am trying to do combine two plots onto the one graph, but I am having a problem with one of the plots. The plots by themselves are fine but when I combine them the cubic plot goes wonky. I had a look at the data for the cubic plot and it seems fine to me - it is not oscillating. I went and changed all my subscripts so that they were unique for each plot and still I get the same problem. Can anybody please shed some light on this problem. It is probably really small and stupid, but somethimes those things are the hardest to find!!!

Cheers,
Matt