Graph showing degrees C in the legend for a degrees F dataset

This sensor code is converting the supplied data in degrees C to degrees F as it should. My understanding with my default units set to Imperial the conversion should take place and it does, but the graph legend is just wrong.

  - platform: mqtt
    name: "Temperature1"
    state_topic: "sensor/temp1"
    qos: 0
    unit_of_measurement: "°C"
#    value_template: "{{ (((states('sensor.temperature1')) * 9 / 5 )  +  32) | float | round(1) }}"
    value_template: '{{ value | float-7 | round(1) }}'

But my graphs are reporting a legend of degrees C with data scaled for degrees F.

If I change the C to F, my data reverts to displaying and graphing degrees C.

What am I doing wrong?

Thanks Matt