If you reverse the X and Y axes, a sigmoid curve:

http://en.wikipedia.org/wiki/Sigmoid_function

seems to fit the data rather well:

And now I am embarrassed again. When I saw this original post, I mistakenly assumed it was an example of a Sigmoid function. I didn't realize it was a fit of *my* data. This particular function, aside from being elegant, transposable, and easily implemented, may actually lead to some insight into the physics of the project I am working on.

 

The Electrician,

I am trying to duplicate your results in TableCurve 2D, but I cannot seem to find any Sigmoidal functions. Am I missing something?

Edit: Got it! It's in the Custom Equation Set. But the definition is just Sigmoid(A,B,C,D) and it doesn't give me the equation. Know why?

 

BTW, TableCurve 2D, I now think, is the most awesome piece of software I ever used, besides a spreadsheet. Well worth the half large they are asking for it!

It even writes code for you.

 

The Electrician,

I am trying to duplicate your results in TableCurve 2D, but I cannot seem to find any Sigmoidal functions. Am I missing something?

Edit: Got it! It's in the Custom Equation Set. But the definition is just Sigmoid(A,B,C,D) and it doesn't give me the equation. Know why?

I used an older version to produce the graph I posted. There have been some changes in the newer versions. You may have to note the equation number and then look through the full list of equations which should be available in the menu bar.