code: def x_frame2D(X,plot_limits=None,resolution=None):
      
      assert X.shape[1]==2, "x_frame2D is defined for two-dimensional inputs"
      if plot_limits is None:
          xmin, xmax = X.min(0), X.max(0)
          xmin, xmax = xmin-0.075*(xmax-xmin), xmax+0.075*(xmax-xmin)
      elif len(plot_limits) == 2:
          xmin, xmax = plot_limits
          try:
              xmin = xmin[0], xmin[1]
          except:
              # only one limit given, copy over to other lim
              xmin = [plot_limits[0], plot_limits[0]]
              xmax = [plot_limits[1], plot_limits[1]]
      elif len(plot_limits) == 4:
          xmin, xmax = (plot_limits[0], plot_limits[2]), (plot_limits[1], plot_limits[3])
      else:
          raise ValueError("Bad limits for plotting")
  
      resolution = resolution or 50
      xx, yy = np.mgrid[xmin[0]:xmax[0]:1j*resolution,xmin[1]:xmax[1]:1j*resolution]
      Xnew = np.c_[xx.flat, yy.flat]
      return Xnew, xx, yy, xmin, xmax
docs: Internal helper function for making plots, returns a set of input values to plot as well as lower and upper limits