def get_ticks(coords, chrom="chrom"):
    """ identify the positions for tick marks for chromosomes
    
    Args:
        coords: pandas DataFrame containing rows with genome coordinates
        chrom: column name for chromosome labels
    
    Returns:
        list of positions for chromosome tick marks.
    """
    
    ticks = []
    for key in sorted(set(coords[chrom])):
        rows = coords[chrom] == key
        pos = coords["pos"][rows]
        midpoint = float(min(pos[rows]) + max(pos[rows]))/2
        ticks.append(midpoint)
    
    return ticks