def _transpose(in_data, keys, field):
    """Turn a list of dicts into dict of lists

    Parameters
    ----------
    in_data : list
        A list of dicts which contain at least one dict.
        All of the inner dicts must have at least the keys
        in `keys`

    keys : list
        The list of keys to extract

    field : str
        The field in the outer dict to use

    Returns
    -------
    transpose : dict
        The transpose of the data
    """
    out = {k: [None] * len(in_data) for k in keys}
    for j, ev in enumerate(in_data):
        dd = ev[field]
        for k in keys:
            out[k][j] = dd[k]
    return out