SUM in Statistics graph card


I have a sensor that shows me the daily running time of our underfloor heating:

  - platform: template
        value_template: "{{ state_attr('climate.smart_thermostat_2109301685957490857248e1e978bea9','hvac_action')}}"
  - platform: history_stats
    name: Heizung Esszimmer Laufzeit
    entity_id: sensor.heizung_esszimmer_on_off
    state: "heating"
    type: time
    start: "{{ now().replace(hour=0, minute=0, second=0) }}"
    end: "{{ now().replace(hour=23, minute=59, second=59) }}"
    ### end: "{{ now() }}"

I can create a Statistics Graph Card from this sensor.
The problem is that I would like to know how long the heating runs in total per month. For this I would probably need the parameter SUM, but unfortunately it is grayed out. All other values such as MIN, MAX and MEAN work.
Why can’t HA add up the values and how can I change this so that it works?

Thank you very much.