You need to really explain more about the characteristics of your curve - but you can always try to fit a reciprocal power series, or basically just fit any function. You could use a circle, or whatever, it all depends on how you want the rate of change of the function with respect to time as it approaches maturity.
Anyway, it wouldn't be a problem whatsoever to get you a function, we just need to chat and work it out.
It should take no more than an hour or two.
Thanks,
John
ps: I have a PhD in physics btw.
pps: Send me a message asap before your repost is deleted. :)
pps: There is a really obvious function that will that graph, with an adjustable paraemter that changes how close the graph "hugs" the axis.