Figure is not updated after the callback

I am struggling to understand why the following code is not updating the figure. My intention is to add a horizontal line to the chart if a particular condition is satisfied, but for some reason, the code below doesn’t update the figure. I would appreciate any help on this issue.

import dash
import dash_core_components as dcc
import dash_html_components as html
import as px
import plotly.graph_objects as go
from dash.dependencies import Input, Output, State
from dash.exceptions import PreventUpdate

if __name__ == "__main__":
    app = dash.Dash()
    layout = [dcc.Graph(id="graph", figure=px.scatter(),
                        style={"height": "75vh"}),
    app.layout = html.Div(children=layout)
    flag = False

    @app.callback(Output("graph", "figure"),
                  Input("ms-beat", "n_intervals"),
                  State("graph", "figure"))
    def add_level_lines(n_intervals, figure):
        global flag
        if flag:
            raise PreventUpdate
        flag = True
        figure = go.Figure(figure)
        figure.add_hline(y=42, line_dash="dot", annotation_text="label")
        return figure

    # app.run_server()