code: def plot2D_samples_mat(xs, xt, G, thr=1e-8, **kwargs):
      
      if ('color' not in kwargs) and ('c' not in kwargs):
          kwargs['color'] = 'k'
      mx = G.max()
      for i in range(xs.shape[0]):
          for j in range(xt.shape[0]):
              if G[i, j] / mx > thr:
                  pl.plot([xs[i, 0], xt[j, 0]], [xs[i, 1], xt[j, 1]],
                          alpha=G[i, j] / mx, **kwargs)
docs: Plot matrix M  in 2D with  lines using alpha values
  
      Plot lines between source and target 2D samples with a color
      proportional to the value of the matrix G between samples.
  
  
      Parameters
      ----------
      xs : ndarray, shape (ns,2)
          Source samples positions
      b : ndarray, shape (nt,2)
          Target samples positions
      G : ndarray, shape (na,nb)
          OT matrix
      thr : float, optional
          threshold above which the line is drawn
      **kwargs : dict
          paameters given to the plot functions (default color is black if
          nothing given)