def _plot_unit_traces(ax, x, data, ci, color, err_kws, **kwargs):
    """Plot a trace for each observation in the original data."""
    if isinstance(color, list):
        if 'alpha' not in err_kws:
            err_kws['alpha'] = 0.5
        for i, obs in enumerate(data):
            ax.plot(x, obs, color=color[i], label='_nolegend_', **err_kws)
    else:
        if 'alpha' not in err_kws:
            err_kws['alpha'] = 0.2
        ax.plot(x, data.T, color=color, label='_nolegend_', **err_kws)
