In reply to your deleted answer from yesterday, your error was, as Jan suggested, that you were confusing x/y with row/col. For image processing in general I (and others) highly recommend getting in the habit of using the first and second dimension - naming your variables r/c instead of x/y etc.
This is a working solution to your problem yesterday:
% Obtains the points which you want the slope between:
disp('Please pick the two points of the line profile (pick upper point first): ');
imshow(image_vec);
title('Please pick the two points of the line profile (pick upper point first): ');
xlabel('x pixel');
ylabel('y pixel');
[x,y] = ginput(2);
close all
% slope of the line between point A and B
m = diff(x)/diff(y); %note switch
b = x(1)-m*y(1);
% Function coordinates f(x) = y
fline = @(pix)pix.*m+b;
xPixel = 1:size(image_vec,2);
yPixel = fline(xPixel);
figure
imshow(image_vec);
hold on;
plot(yPixel,xPixel,'g'); %note switch!
title('Uploaded Image');
xlabel('x-pixel'); ylabel('y-pixel');
If you want the value on the line - use round (or fix / ceil) and sub2ind to get the values on the image.
