How to Test without a screen

[GraemeDBlue]

Is your feature request related to a problem? Please describe.
Wonderful project, I'm just trying to add my own tariff FLUX-IMPORT-23-02-14/ but trying to do it on a RPi Zero is soo slow, so would like to know the best way to just test this without the Inky pHat dependencies on my host machine. As I'm having issues adding the tarrif getting

Connected to database...
Inky pHAT display selected.
Standard display orientation.
Working in Octopus Agile import mode.
Agile version set: 40p cap Flux)
Detected Black pHAT (SSD1608)
Traceback (most recent call last):
  File "/home/pi/pi-eco-indicator/./update_display.py", line 67, in <module>
    eco_indicator.update_inky(config, data_rows, args.demo)
  File "/home/pi/pi-eco-indicator/eco_indicator.py", line 372, in update_inky
    high_slots_start_idx = high_slots_list.index(max(high_slots_list))
ValueError: max() arg is an empty sequence

Describe the solution you'd like
Option to have the output as the Inky, Blinkt or terminal ( for development )

Describe alternatives you've considered
I'm trying to work through it and have forked your repo here https://github.com/GraemeDBlue/pi-eco-indicator but if I can would like to contribute back to this repo.

Additional context
Great little project got me up and running in no time, fantastic docs.