Where does the seven come from?

Here's a plot of exp(6it)/2 + exp(20it)/3:

Notice that the plot has 7-fold symmetry. You might expect 6-fold symmetry from looking at the equation. Where did the 7 come from?

I produced the plot using the code from this post, changing the line defining the function to plot to

 def f(t): return exp(6j*t)/2 + exp(20j*t)/3

You can find the solution in Eliot's comment in this Twitter thread.

• Daily exponential sum
• Mystery curve