Communication between callbacks? "Example 2 - Computing Aggregations Upfront"

Dash Python
1

Hello,

“Example 2 - Computing Aggregations Upfront” on Part 5. Sharing Data Between Callbacks | Dash for Python Documentation | Plotly appeared to be a bit confusing to me:

@app.callback(
    Output('intermediate-value', 'children'),
    Input('dropdown', 'value'))
def clean_data(value):
     # an expensive query step
     cleaned_df = your_expensive_clean_or_compute_step(value)

     # a few filter steps that compute the data
     # as it's needed in the future callbacks
     df_1 = cleaned_df[cleaned_df['fruit'] == 'apples']
     df_2 = cleaned_df[cleaned_df['fruit'] == 'oranges']
     df_3 = cleaned_df[cleaned_df['fruit'] == 'figs']

     datasets = {
         'df_1': df_1.to_json(orient='split', date_format='iso'),
         'df_2': df_2.to_json(orient='split', date_format='iso'),
         'df_3': df_3.to_json(orient='split', date_format='iso'),
     }

     return json.dumps(datasets)

@app.callback(
    Output('graph', 'figure'),
    Input('intermediate-value', 'children'))
def update_graph_1(jsonified_cleaned_data):
    datasets = json.loads(jsonified_cleaned_data)
    dff = pd.read_json(datasets['df_1'], orient='split')
    figure = create_figure_1(dff)
    return figure

@app.callback(
    Output('graph', 'figure'),
    Input('intermediate-value', 'children'))
def update_graph_2(jsonified_cleaned_data):
    datasets = json.loads(jsonified_cleaned_data)
    dff = pd.read_json(datasets['df_2'], orient='split')
    figure = create_figure_2(dff)
    return figure

@app.callback(
    Output('graph', 'figure'),
    Input('intermediate-value', 'children'))
def update_graph_3(jsonified_cleaned_data):
    datasets = json.loads(jsonified_cleaned_data)
    dff = pd.read_json(datasets['df_3'], orient='split')
    figure = create_figure_3(dff)
    return figure
  1. Why is Output('graph', 'figure') used in 3 callbacks? I thought one output can only be used once. Otherwise I get an error.
  2. Isn’t there a html element with the id 'intermediate-value' necessary? Doesn’t work for me without a hidden element.

In general, the strategy with hidden html elements to trigger a callback from another one seems a bit dirty to me. Is there a possibility to trigger a callback which e.g. spawns a modal when another callback is finished without using a html element in between?

Thank you!