FlexLine: labels attribute not working #1272

The "labels" attribute doesn't display for FlexLine mark.

To Reproduce
from bqplot import *

Get Data

dates = np.arange('2005-02', '2005-03', dtype='datetime64[D]')
size = len(dates)
spx = 100 + 5 * np.cumsum(np.random.randn(size))
vix = 10 + np.cumsum(np.random.randn(size))

Displaying extra dimension with color

lin_x = DateScale()
lin_y = LinearScale()
col_line = ColorScale(colors=['green', 'white', 'red'])

ax_x = Axis(scale=lin_x, label='Date', label_location='end')
ax_y = Axis(scale=lin_y, orientation='vertical', label='Index', label_offset='4ex')
ax_col = ColorAxis(label='Vol', scale=col_line, tick_format='0.2f')

fig_margin = dict(top=50, left=80, right=20, bottom=70)
fl = FlexLine(x=dates, y=spx, color=vix,
scales={'x': lin_x, 'color': col_line, 'y': lin_y},
labels=['TEST_LABEL'], display_legend=True)

Figure(marks=[fl], axes=[ax_x, ax_y, ax_col], fig_margin=fig_margin)