[ Closed ] bjerrum diagram

The example graph looks as a plot of your function but reversed: x=a+log((1-y)/y). Hence: f(x)=1/(1+10^(x-a));
But I have no idea what the Bjerrum diagram is.

ok i tryed to make a general funktion (custom funktion)
1+(-$b/(1+$d^(x-$c)))
but then graf come with a error and will not draw a graf

then i tryed 3 other funktions
1+(-$b/(1+10^(x-$c))) (the blue line)
1+(-$b/(1+8^(x-$c)))   (red line)
1+(-$b/(1+1.5^(x-$c))) (purple line)

and i get what you see on the picture, the funktion with 1.5^ is far better then the other 2, but dosent fit perfectly.

now i ask is there a way to make graph draw a line to make it fit the graph (change the potens)

"I suppose you inserted your general function as a User defined model into "Insert trendline" dialog. By default, initial values for your constants are $b=1, $c=1, $d=1, very much different from resulting figures."
you are correct, i did do that.

from our raw data I can easily find the $b value

but here is the problem
i need to find the $c value from the best possible graph aka. R^2=1 (or very close)
and i find the best graph by finding the best $d value. with is a little different from every experiment we made
and even with only 2 variables it still come with an error

i can inset random numbers until i get close but it will take to long.
you say "The program runs 100 iterations to find reasonable results", can i force the program to make more?

If 100 iterations are not enough, 10000 iterations will probably not help. Instead you have to change the default guess. After entering your model, here 1+(-$b/(1+$d^(x-$c))), you need to use the Add model button. Now you can give the model a name, e.g. Bjerrum and change the default guess. Graph will suggest 1 as the default guess, but that will not work here. Instead you can try with $b=1, $d=1.5, $c=50.