This one took some doing. The line graph is simply plotted from the equation for single slit diffraction.

If you run the animation you can see where that equation comes from, It's the square of the resultant amplitude found by adding the amplitudes from a large number of point sources. If you show resultant² you will see a series of blue dots that represent the intensity, these dots are calculated from the rotating vectors not the equation (honest). To get the two lines to match wasn't easy. Note that there are no units on the values and the diagram is not to scale but it should help to visualise the concept.