When a unit_of_measurement exists, Home Assistant assumes it's an integer and tries to graph it. Even if no unit_of_measurement are given. By not adding them, Home Assistant takes the value as discrete and visualizes it accordingly.
It's only relevant for few registers. I am using it for "work_state_1". What the broken graph in Home Assistant looks like at the moment: https://imgur.com/JB67fax. After removing units_of_measurement: https://imgur.com/6rYNAAA and combined with other data: https://imgur.com/1TzAqFd. work_state_1 shows not only "Run" and "Standby", but "Starting" and "Degraded Run" as well which is important for monitoring.
Luca Hammer