code: def add_arrows(axes):
      """Add some arrows at 60 Hz and its harmonics."""
      for ax in axes:
          freqs = ax.lines[-1].get_xdata()
          psds = ax.lines[-1].get_ydata()
          for freq in (60, 120, 180, 240):
              idx = np.searchsorted(freqs, freq)
              # get ymax of a small region around the freq. of interest
              y = psds[(idx - 4) : (idx + 5)].max()
              ax.arrow(
                  x=freqs[idx],
                  y=y + 18,
                  dx=0,
                  dy=-12,
                  color="red",
                  width=0.1,
                  head_width=3,
                  length_includes_head=True,
              )
  
imports: import os
  import matplotlib.pyplot
  import numpy
  import mne
label_desc: Use image node_id 1 for calling a global function with example usage: add_arrows(axes) without return types