code: def test_grayscale_alpha():
      """Masking images with NaN did not work for grayscale images"""
      x, y = np.ogrid[-2:2:0.1, -2:2:0.1]
      dd = np.exp(-(x**2 + y**2))
      dd[dd < 0.1] = np.nan
      fig, ax = plt.subplots()
      ax.imshow(dd, interpolation="none", cmap="gray_r")
      ax.set_xticks([])
      ax.set_yticks([])
  
imports: import datetime
  import decimal
  import io
  import os
  from pathlib import Path
  import numpy
  import pytest
  import matplotlib
  from matplotlib import pyplot, rcParams, font_manager
  from matplotlib.cbook import _get_data_path
  from matplotlib.ft2font import FT2Font
  from matplotlib.font_manager import findfont, FontProperties
  from matplotlib.backends._backend_pdf_ps import get_glyphs_subset
  from matplotlib.backends.backend_pdf import PdfPages
  from matplotlib.patches import Rectangle
  from matplotlib.testing.decorators import check_figures_equal, image_comparison
  from matplotlib.testing._markers import needs_usetex
label_desc: Use image node_id 16 for calling a global function with example usage: test_grayscale_alpha() without return types