zabbix avg calculation wrong when measurement interval is not constant

Steps to reproduce:

1. I collect data from a MQTT power measuring device
2. The MQTT power measuring device does not send data at a regular interval but when it detects a significant change in power consumption
3. in this case this leads to more measurements when the power consumption is low

Result:

The calculated avg is 1.35 KW for the last 1 hour

See screenshot attached

Expected:

Since power consumption is 2.61 KW 80% of the time and 0.55 KW 20% of the time, the calculated avg for the last 1h should be about 2.18 KW

I think this happens because Zabbix calculates the Avg by just summing up all values and dividing through the number of values taken.

But it does not take into account the time interval between each measurement taken.

This method only works if the measurement values are taken at an exactly regular interval, say each 1 minute