What is going on in this plot?

Question

Plot[{ArgMax[Cos[t] Sinc[2*Pi kl (Sin[t] - 1/Sqrt[2])], t]*180/Pi, 
  45}, {kl, 0, 10}, PlotRange -> Full]

I want to know what causes this, and how to fix it in case it happens in the future.

Answer 1

As noted by kirma, this function is highly oscillatory. However, from its form it is periodic in t with period 2 Pi and has its maximum near Pi/4 except for small kl. For instance,

Plot[Evaluate[Cos[t] Sinc[2 Pi kl (Sin[t] - 1/Sqrt[2])] /. 
    kl -> Range[0, 2, 1/2]], {t, 0, 2 Pi}, PlotRange -> All]

Because ArgMax sometimes finds a local maximum instead of the global maximum, we can help it by adding a constraint that takes advantage of the fact (just shown) that the maximum lies between 0 and Pi/4.

Plot[{ArgMax[{Cos[t] Sinc[2*Pi kl (Sin[t] - 1/Sqrt[2])], 
    0 < t < Pi/2}, t]*180/Pi, 45}, {kl, 0, 10}, PlotRange -> Full]

as desired.