Showing how to make a Bokeh stacked area with hover labels per-series

stacked_area_hover.py

def stacked_area(df, colours, series_noun, **kwargs):
    """Return a stacked area plot."""
    columns = df.columns
    areas = stacked(df, columns)
    x2 = np.hstack((df.index[::-1], df.index))

    p = figure(
        width=900, height=600, tools=[
            SaveTool(),
            HoverTool(tooltips=[(series_noun, "@name")])
        ],
        x_axis_type="datetime",
        **kwargs
    )
    p.grid.minor_grid_line_color = '#eeeeee'

    ys = []
    xs = []
    names = []

    for col in columns:
        ys.append(areas[col])
        xs.append(x2)
        names.append(col)

    source = ColumnDataSource(data=dict(
        x=xs,
        y=ys,
        name=names,
    ))

    p.patches(
        'x', 'y', source=source, color=colours
    )
    p.xaxis.formatter = DatetimeTickFormatter(
        formats=dict(days=["%a\n%d %b"])
    )
    return p

whta is stacked here? where is it imported from?stacked(df, columns)