There should already be a sensor that will provide the daily total for both gas and electricity cost and consumption if you wish.
Something like:
Daily Electricity Cost Total
sensor.octopus_energy_electricity_xxxxxx_xxxxxxxxx_current_accumulative_cost
Daily Gas Cost Total
sensor.octopus_energy_gas_xxxxxx_xxxxxxxxx_current_accumulative_cost
Daily Gas Comsumption Total (m3)
sensor.octopus_energy_gas_xxxxxx_xxxxxxxxxx_current_accumulative_consumption_m3
Daily Gas Comsumption Total (Kwh)
sensor.octopus_energy_gas_xxxxxx_xxxxxxxxxx_current_accumulative_consumption_kwh
Daily Electricity Consumption Total
sensor.octopus_energy_electricity_xxxxxx_xxxxxxxxxx_current_accumulative_consumption
Create an entry in the dashboard as a sensor, gauge or statistical graph and then you can drill in to it and it will plot historically.