def plot_trajectories(T, colors=None, fig='current', ax=None, colorbar=False,
                      cmap=None, alpha=1, linewidth=1, title=None):
  '''Plot lines in T as trajectories (2d only).'''
  ax = _get_axis(fig, ax, False)
  if colors is not None:
    colors = np.asanyarray(colors)
  lc = LineCollection(T, array=colors, cmap=cmap, alpha=alpha,
                      linewidth=linewidth)
  ax.add_collection(lc, autolim=True)
  if colors is not None and colorbar:
    cbar = ax.figure.colorbar(lc)
    cbar.set_alpha(1.)  # colorbars with alpha are ugly
    cbar.draw_all()
  ax.autoscale_view()
  if title:
    ax.set_title(title)
  return plt.show