def METHOD_NAME(doc):
    source = ColumnDataSource(dict(x=[1, 2], y=[1, 1], val=["a", "b"]))
    plot = Plot(height=400, width=400, x_range=Range1d(0, 1), y_range=Range1d(0, 1), min_border=0)
    plot.add_glyph(source, Circle(x='x', y='y', size=20))
    plot.tags.append(CustomJS(name="custom-action", args=dict(s=source), code=RECORD("data", "s.data")))
    def cb(attr, old, new):
        source.data['val'] = ([*group.active, 0, 0])[:2] # keep col length at 2, padded with zero
    group.on_change('active', cb)
    doc.add_root(column(group, plot))