If you want to do it purely by eye, I would think that you could do it with Gimp. Start with the first image, enlarge its canvas horizontally as much as necessary, then copy and paste the other images, dragging each one into position after pasting it. (You have to flatten the image after enlarging its canvas, otherwise pasting doesn't seem to do anything).
That being said, I would have thought that Hugin would do the job, but it has so many expert options that it's a bit overwhelming.