def qqline(ax, line, x=None, y=None, dist=None, fmt='r-'):
    if line == '45':
        end_pts = lzip(ax.get_xlim(), ax.get_ylim())
        end_pts[0] = min(end_pts[0])
        end_pts[1] = max(end_pts[1])
        ax.plot(end_pts, end_pts, fmt)
        ax.set_xlim(end_pts)
        ax.set_ylim(end_pts)
        return # does this have any side effects?
    if x is None and y is None:
        raise ValueError("If line is not 45, x and y cannot be None.")
    elif line == 'r':
        # could use ax.lines[0].get_xdata(), get_ydata(),
        # but don't know axes are 'clean'
        y = OLS(y, add_constant(x)).fit().fittedvalues
        ax.plot(x,y,fmt)
    elif line == 's':
        m,b = y.std(), y.mean()
        ref_line = x*m + b
        ax.plot(x, ref_line, fmt)
    elif line == 'q':
        _check_for_ppf(dist)
        q25 = stats.scoreatpercentile(y, 25)
        q75 = stats.scoreatpercentile(y, 75)
        theoretical_quartiles = dist.ppf([0.25, 0.75])
        m = (q75 - q25) / np.diff(theoretical_quartiles)
        b = q25 - m*theoretical_quartiles[0]
        ax.plot(x, m*x + b, fmt)


#about 10x faster than plotting_position in sandbox and mstats
Plot a reference line for a qqplot.
dataset/ETHPy150Open statsmodels/statsmodels/statsmodels/graphics/gofplots.py/qqline