code: def boundary(ax, scale, axes_colors=None, **kwargs):
      """
      Plots the boundary of the simplex. Creates and returns matplotlib axis if
      none given.
  
      Parameters
      ----------
      ax: Matplotlib AxesSubplot, None
          The subplot to draw on.
      scale: float
          Simplex scale size.
      kwargs:
          Any kwargs to pass through to matplotlib.
      axes_colors: dict
          Option for coloring boundaries different colors.
          e.g. {'l': 'g'} for coloring the left axis boundary green
      """
  
      # Set default color as black.
      if axes_colors is None:
          axes_colors = dict()
      for _axis in ["l", "r", "b"]:
          if _axis not in axes_colors.keys():
              axes_colors[_axis] = "black"
  
      horizontal_line(ax, scale, 0, color=axes_colors["b"], **kwargs)
      left_parallel_line(ax, scale, 0, color=axes_colors["l"], **kwargs)
      right_parallel_line(
          ax, scale, 0, color=axes_colors["r"], **kwargs
      )
      return ax
  
file_path: python-ternary/ternary/lines.py
imports: from numpy import arange
  from matplotlib.lines import Line2D
  from .helpers import project_point
label_desc: Use image node_id 5 for calling a global function with example usage: boundary(ax, scale, axes_colors) and returns: ax