Help With A Line Graph In Excel

I wish to construct a line graph using Microsoft Excel that will have two X axis. I wish to show three cost per ton lines and also show the tonnage on a month by month basis.

I have been able to construct the graph showing the three costs per ton with the figures on the left hand axis, however, when I introduce the tonnage data with the figures on the right hand axis one of the lines from the cost per ton data falls of the graph!!

Is there a limit to just three lines on a line graph or can more be introduced, if so how ??

Any help would be most appreciated.
David Jarvis