@Jake
\addplot+[raw gnuplot, draw=red, mark=none, smooth] gnuplot {
f(x) = a*x/(b+x);
a=0;
b=0;
set fit errorvariables;
fit f(x) 'source6.dat' using 1:4 via a,b;
% Next, plot the function and specify plot range
% The range should be approx. the same as the test.dat x range
plot [x=0.1:1000] f(x);
set print "parameters.dat"; % Open a file to save the parameters into
print a, a_err; % Write the parameters to file
print b, b_err;