'create scatter plot of two random effects
Parameters
idx1, idx2 : int
indices of the two random effects to display, corresponding to
columns of exog_re
title : None or string
If None, then a default title is added
ax : None or matplotlib axis instance
If None, then a figure with one axis is created and returned.
If ax is not None, then the scatter plot is created on it, and
this axis instance is returned.
Returns
ax_or_fig : axis or figure instance
see ax parameter
Notes
Still needs ellipse from estimated parameters'

def plot_scatter_pairs(self, idx1, idx2, title=None, ax=None):

    import matplotlib.pyplot as plt
    if (ax is None):
        fig = plt.figure()
        ax = fig.add_subplot(1, 1, 1)
        ax_or_fig = fig
    re1 = self.params_random_units[:, idx1]
    re2 = self.params_random_units[:, idx2]
    ax.plot(re1, re2, 'o', alpha=0.75)
    if (title is None):
        title = ('Random    Effects    %d    and    %d' % (idx1, idx2))
    ax.set_title(title)
    ax_or_fig = ax
    return ax_or_fig
