TITLE: What equation produces this curve?
QUESTION [23 upvotes]: I'm working on an engineering project, and I'd like to be able to input an equation into my CAD software, rather than drawing a spline.
The spline is pretty simple - a gentle curve which begins and ends horizontal.
Is there a simple equation for this curve?
Or perhaps two equations, one for each half?
I can also work with parametric equations, if necessary.

REPLY [3 votes]: I am an professional electrical engineer (according to the gubbermint of California), so I will give you an engineer's answer that is sure to be offensive to the math people here.  
Download a program called "Curve Expert"  Just get the basic program, not the professional version.   Input your data, and you will get a better fit than you want.   From your graph, it could be a sinusoid, a polynomial, an inverse hyperbolic sine, a rational function, something built from the cumulative normal distribution, or some kind of logistic function.   
Curve expert will help you decide what is best.  You will probably fall in love with it and buy the license when the trial period expires.
*** disclaimer: I have used curve expert for years, but have no financial, romantic, political, or other interest in it.