One way to do this would be to calculate the smallest common denominator of the distances between points on the X axis. You could then use that to make the X-values equally spaced. As for those X-values that then don't have a value, use simple triangulation.

I hope I make myself reasonably clear. This is one of the moments where I would just make a small drawing in real life. If my explanation doesn't seem to make sense, drop me a line and I'll put some sort of drawing up on my webpage.

Brother Ade