Implementing least squares with known constants to improve fit

[Mdhiggenz]

Homework Statement

Hello all,

So I am working on my engineering project, and my professor told us to play with the graphs,and try to find something interesting about them.

So what I want to do is form a better fit for the figure below. " figure 2b" , and find out why the choose not to do a better fit.
However I am given all the constants " d1,d2,ca2+ " so I am not sure how to implement least squares in this situation.

Help would be appreciated

Homework Equations

The Attempt at a Solution

 

[mfb]

Those data points look ill, or there are error bars missing. I would try something like a (shifted) arctan, a logistic function or the error function.