How to add a trend plane in a 3D scatter plot?

[GtHbbrr]

Hi there, I managed to create a 3D scatter plot and now I would like to add a trend plane to this plot.
Can anybody point me in the right direction on how to get this done?

This is the code for the #D scatter plot:

type: custom:plotly-graph
title: COP vs Tlift (Ta - Buitentemp)
hours_to_show: 960
refresh_interval: 300
no_default_entities: true
raw_plotly_config: true
entities:
  - entity: sensor.heatpump_1_temperature_water_in
    internal: true
    filters:
      - resample: 1m
      - store_var: ta
  - entity: sensor.temperatuur_knmi
    internal: true
    filters:
      - resample: 1m
      - store_var: buiten
  - entity: sensor.heatpump_1_quatt_cop
    internal: true
    filters:
      - resample: 1m
      - store_var: cop
  - entity: ""
    x: >-
      $fn ({ vars }) => vars.cop.xs.map((x, i) => vars.ta.ys[i] -
      vars.buiten.ys[i])
    z: $fn ({ vars }) => vars.cop.ys
    "y": $fn ({ vars }) => vars.buiten.ys
    type: scatter3d
    mode: markers
    marker:
      color: crimson
      size: 2
      opacity: 0.1
layout:
  margin:
    t: 0
    l: 0
    r: 0
    b: 0
  scene:
    xaxis:
      title: Tlift = Ta - Buitentemp
      range:
        - 0
        - 25
    zaxis:
      title: COP
      range:
        - 1
        - 10
    yaxis:
      title: Buitentemp
      range:
        - 10
        - 16
  height: 600
  plot_bgcolor: white
  hovermode: closest