Whats wrong with this simpleGaussian Fit?

For some reason the fit doesn't fit!

%Gaussian Fit--------------------------------------------------------
gaussEqn = 'a*exp(-((x-b)/c).^2)+d'
f = fit(xdata',ydata',gaussEqn)
%Results of fit
coeffs=coeffvalues(f);
a=coeffs(1)
b=coeffs(2)
c=coeffs(3)
d=coeffs(4)
figure
plot(xdata',ydata','r.')  %plot raw data
hold on;

%Increase resolution of x data
xdataFine=(linspace(xdata(1),xdata(end),30))  
%plot high res fit
fitGaus =  a*exp(-((xdataFine-b)/c).^2)+d
plot(fitGaus,'g-')
hold off.

[Jason]

It seems the parameters fromt he fit are wrong. This is my data

and here are the results of the fit

f =

     General model:
     f(x) = a*exp(-((x-b)/c).^2)+d
     Coefficients (with 95% confidence bounds):
       a =      0.4218
       b =      0.9157
       c =      0.7922
       d =        3351  (-646.9, 7349)

"b" should be close the 7 so I can't figure out whats going wrong with the fit, and then to plot the fit as well.

It fits fine in Mathcad.

[Jason]

Ok, it seems like its not possible to do the fit without using start guesses:

f = fit(x,y,gaussEqn,'Normalize','off', 'StartPoint',[max(y(:))-min(y(:)),xValue,c,min(y(:))]);

Answer 1

[the cyclist]

Those red circles look like they are the result of binning and counting the data (and making a histogram-style figure). Are those counts what you are putting into the fit? Most fitting routines in MATLAB expect the underlying data, not counts.

fit points to several functions in MATLAB, depending on context. Which function is it?

[Jason]

Hi, no the data is not a histogram, its a line scan across an image of a bright spot. So its real data!

The fit only points to one function:

gaussEqn = 'a*exp(-((x-b)/c).^2)+d'
f = fit(xdata',ydata',gaussEqn)

[the cyclist]

You misunderstood my last question. When you type

which fit

what MATLAB function comes up? (I'm trying to understand which function to look at to help you diagnose the issue.)

[Jason]

Im sorry. It comes up with:

C:\Program Files\MATLAB\R2014b\toolbox\curvefit\curvefit\fit.m