Reading data from file into sensor card

Hi, I am totally new to HA. I have been trying to read my solar data from a file and put into a dashboard Sensor Card so it is graphed.
I have a new installation of HA with only HACS added.
The following is the format in a solar log file which is updated every 5 mins with a bash script:
Date,Time,Amp PV1,Amp PV2,V PV1, V PV2, W PV1, W PV2, kW today, kW total,…

I can modify the data to any format if HA cant read a coma delimited file.

As a test (and after reading for hours getting started, help and forum post) I have been trying to get just basic info onto a sensor card without success.
In configuration.yaml I have:

  - platform: file
    name: temp
    file_path: /config/externaldata/temp.txt
    value_template: '{{ value_json.temperature }}'
    unit_of_measurement: "°C"

In /config/externaldata/temp.txt it have:

{"temperature": 21, "humidity": 39}
{"temperature": 22, "humidity": 36}

I have added a sensor card on the dashboard but I cant find how to associate the data with a card - how to get the card to display the data - there is no errors in the log file.
If I can just get the temp card to work the solar stuff should be reasonably easy - hopefully.

Any help would be most welcome.