To add a fitted line to a specific subplot, you can just use the "Basic Fitting Tool" in MATLAB. This can be accessed by going to the "Basic Fitting" option in the "Tools" menu on your figure. Equivalently, you can also plot the line directly on your specific subplot programmatically. For example, if your data was x and y:
p = polyfit(x,y,1)
% polyfit(x,y,n) finds the coefficients of a polynomial p(x) of degree n
% that fits the data. This example demonstrates linear trend.
yLin = polyval(p,x);
% polyval(p,x) returns the value of a polynomial of degree n evaluated at
% x.
subplot(3,2,6)
plot(x,y)
hold on
plot(x,yLin,'r');
Hope this helps!
