Prepare a dataframe for graphing by calculating deltas for
    series that need them, resampling, and removing outliers.

def prep_for_graph(data_frame, series=None, delta_series=None, smoothing=None,
                   outlier_stddev=None):
    """Prepare a dataframe for graphing by calculating deltas for
    series that need them, resampling, and removing outliers.
    """
    series = series or []
    delta_series = delta_series or []
    graph = calc_deltas(data_frame, delta_series)

    for s in series + delta_series:
        if smoothing:
            graph[s] = graph[s].resample(smoothing)
        if outlier_stddev:
            graph[s] = remove_outliers(graph[s], outlier_stddev)

    return graph[series + delta_series]