Question: And can you give me an example of an algorithm?
[alt text http://ryancalderoni.com/archive/ideal\_curve.jpg](http://ryancalderoni.com/archive/ideal_curve.jpg)

EDIT: And then how would I calculate the math using Javascript? Can someone add that? Sorry to not include that context originally..

NOTE: I am using 'flot' to graph it and the input for flot is a javascript array like this:

[[x,y],[x,y],[x,y]...]

So given the values that change the curve I output all the points to an array with a loop and spit it out to flot to graph.

Answer: 
If you want a single, analytic function, then ArcTangent and Hyperbolic Tangent both have that shape, you just need to shift it over a bit. If you want it to only start at the origin and be flat, look at Exp[-1/x^n], for n>=1. This produces a curve that is exceedingly flat at the origin.
What you are looking for is called Sigmoid function. You can look it up in Wikipedia. Some function that are used for this are error function or logistic function. These are most commonly use for neural networks.