Return the PGFPlots code for circles.

def _draw_circle(data, obj, draw_options):
    """Return the PGFPlots code for circles.
    """
    x, y = obj.center
    ff = data["float format"]
    cont = ("\\draw[{}] (axis cs:" + ff + "," + ff + ") circle (" + ff + ");\n").format(
        ",".join(draw_options), x, y, obj.get_radius()
    )
    return data, cont