def subpx_bias(f, pos_columns=None):
    """Histogram the fractional part of the x and y position.

    Parameters
    ----------
    f : DataFrame
    pos_columns : list of column names, optional

    Notes
    -----
    If subpixel accuracy is good, this should be flat. If it depressed in the
    middle, try using a larger value for feature diameter."""
    if pos_columns is None:
        if 'z' in f:
            pos_columns = ['x', 'y', 'z']
        else:
            pos_columns = ['x', 'y']
    axlist = f[pos_columns].applymap(lambda x: x % 1).hist()
    return axlist