Bokeh checkbox only updates when its checked

Im using bokeh server to plot a line graph, where theres a checkbox button that will flip the line if its checked. If its unchecked I want to see the original version of the line (unflipped). Following the flip/unflip, a second function is called to perform some other calculations:

import numpy as np
from bokeh.io import curdoc
from bokeh.layouts import row, widgetbox, layout
from bokeh.models import ColumnDataSource
from bokeh.models.widgets import CheckboxGroup
from bokeh.plotting import figure


def flip_signal(signal, flip):
    if flip:
        signal = -signal
    else:
        signal = signal

    return signal


N = 200
x = np.linspace(0, 4*np.pi, N)
y = np.sin(x)
source = ColumnDataSource(data=dict(x=x, y=y))

plot = figure(plot_height=500, plot_width=850, title="test",
              tools="crosshair,pan,reset,save,wheel_zoom")
line_orig = plot.line('x', 'y', source=source, line_width=1, line_alpha=1)

flip_signal_btn = CheckboxGroup(labels=["Flip signal"])


def update_flip(attrname, old, new):
    if 0 in flip_signal_btn.active:
        flip = True
    else:
        flip = False

    # Update plot
    source.data = dict(x=source.data['x'], y=flip_signal(source.data['y'], flip))


def update_peaks(attrname, old, new):
    # do something else
    pass  


for w in [flip_signal_btn]:
    w.on_change('active', update_flip)
    w.on_change('active', update_peaks)

options = widgetbox(flip_signal_btn)
doc_layout = layout(row([options], height=200))

curdoc().add_root(row(plot, doc_layout, width=800))
curdoc().title = "checkbox"

The checkbox only seems to call update_flip when its checked, so in order to flip (or unflip) the signal I need to click it twice. For example, when I uncheck the box nothing happens, but I'm expecting it to unflip the signal. Rather it only unflips the signal if I uncheck and then check the box again