Answer: OK, stay with me on this one
the image is what your graph will look like, but with 2 lines instead of one. on the bottom, mark the months. on the side, write temperatures. Then on each month for the line for A, dot the temperature it is with a connected line. (Like the line is on the image.) Then do the same for the line for city B. I'd recommend doing it in 2 different colors, and label some where else which city.
I can go into a little more detail if needed