code: def plot_flat_gamut(
      xy_to_2d=lambda xy: xy,
      axes_labels=("x", "y"),
      plot_rgb_triangle=True,
      fill_horseshoe=True,
      plot_planckian_locus=True,
  ):
      
      observer = observers.cie_1931_2()
      # observer = observers.cie_1964_10()
  
      _plot_monochromatic(observer, xy_to_2d, fill_horseshoe=fill_horseshoe)
      # plt.grid()
  
      if plot_rgb_triangle:
          _plot_rgb_triangle(xy_to_2d)
      if plot_planckian_locus:
          _plot_planckian_locus(observer, xy_to_2d)
  
      plt.gca().set_aspect("equal")
      # plt.legend()
      plt.xlabel(axes_labels[0])
      plt.ylabel(axes_labels[1])
      return
docs: Show a flat color gamut, by default xy.  There exists a chroma gamut for
      all color models which transform lines in XYZ to lines, and hence have a
      natural decomposition into lightness and chroma components.  Also, the flat
      gamut is the same for every lightness value. Examples for color models with
      this property are CIELUV and IPT, examples for color models without are
      CIELAB and CIECAM02.