Subtract one entiry value from another

[plandregan]

I have the below Pie chart, with 4 power items being monitored.
The 5th value is total consumed. What I am trying to do is the 5th item be the total - the 4 devices.
How do you subtract one from another. I was thinking of Transform but it doesnt seem to have any maths functions

type: custom:apexcharts-card
experimental:
color_threshold: true
chart_type: pie
graph_span: 24h
now:
show: false
color: red
label: Now
span:
start: day
series:

• entity: sensor.smartplug_garagefridge_energy_today
  name: Fridge
  color: red
  group_by:
  func: last
  duration: 5m
  stroke_width: 2
  show:
  extremas: false
  header_color_threshold: true
• entity: sensor.summerhouse_energy_today
  name: SummerHouse
  show:
  in_chart: true
  group_by:
  func: last
  duration: 10m
  color: orange
  stroke_width: 2
• entity: sensor.tasmota_energy_today
  name: Dryer
  show:
  in_chart: true
  group_by:
  func: last
  duration: 10m
  color: green
  stroke_width: 2
• entity: sensor.smartplug_pond_energy_today
  name: FishPond
  show:
  in_chart: true
  group_by:
  func: last
  duration: 10m
  color: steelblue
  stroke_width: 2
• entity: sensor.givtcp_self_consumption_energy_today_kwh
  name: Total
  show:
  in_chart: true
  group_by:
  func: last
  duration: 10m
  color: blue
  stroke_width: 2