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.
Might I sugggest:

* The error function (`erf(x)` in C)
* The normal cumulative distribution function (`0.5 * erfc(-x/sqrt(2))`)
* The logistic function (`1.0 / (1.0 + exp(-x))`)
* Any other sigmoid function