def data_by_class(data):
    """
    Organize `data` by class.

    Parameters
    ----------
    data : list of dicts
        Each dict contains the key `symbol_id` which is the class label.

    Returns
    -------
    dbc : dict
        mapping class labels to lists of dicts
    """
    dbc = {}
    for item in data:
        if item['symbol_id'] in dbc:
            dbc[item['symbol_id']].append(item)
        else:
            dbc[item['symbol_id']] = [item]
    return dbc
