def _add_errorbar(ax, x, y, yerr, filled, markersize=None, color=None):
    """Add an errorbar to Axes `ax`, allowing an array of markers."""
    ax.errorbar(x[filled], y[filled], yerr[filled], ls='None', marker='o',
        markersize=markersize, color=color)
    notfilled = ~filled
    ax.errorbar(x[notfilled], y[notfilled], yerr[notfilled], ls='None', mfc
        ='None', marker='o', markersize=markersize, color=color)
