You might find it helpful to plot delivered heat energy per day vs delta, as this will be linear most of the time, excepting very windy or sunny days, and assuming constant indoor temperature. Heat delivered will match heat loss, which is a function of temperature differential, whereas electricity consumed will depend on the coefficient of performance.
It is possible to get real time data out of the Daikin heat pump, with some technical expertise. This topic describes what’s involved: Daikin Altherma, ESPAltherma & Home Assistant with OpenEnergyMonitor