Edit: To explicitly answer your question, the angle of each arrow is
proportional to the height of the graph above that arrow. Note that different
heights on the graph can correspond to identical angles, since (for example) 0
radians, 2pi radians, and 4pi radians are all the same angle.
Feynman talks about this between 59:33 and 60:32 of part one of his 1979 Douglas Robb lectures.
Between 29:41 and 36:27 of part two, he draws the “arrows” diagram on the chalkboard.
If you find this topic interesting, you’ll enjoy all four parts of the lecture series. See also 63:26 to 63:35 of part one, which is relevant to your other question.
Edit: To explicitly answer your question, the angle of each arrow is proportional to the height of the graph above that arrow. Note that different heights on the graph can correspond to identical angles, since (for example) 0 radians, 2pi radians, and 4pi radians are all the same angle.
Thank you very much.