Pie chart + measure?

There are 2 important steps:

1. Add "Idle" as a subevent for each event (in the worst case its values will be 0)
2. Create a dedicated "Duration" calculation for newly created "Idle" row

First we need to obtain this table in Power Query

Then you can use calculated column for the calculation of "idle" duration:

First we need to obtain total event time:

Duration of Event = 
VAR _CurrentEvent = SELECTEDVALUE( Table_Events[Event] )
VAR _StartOfSelectedEvent = 
    MINX(
        FILTER(
            Table_Events,
            Table_Events[Event] = _CurrentEvent
        ),
        Table_Events[Start]
    )

VAR _EndOfSelectedEvent = 
    MAXX(
        FILTER(
            Table_Events,
            Table_Events[Event] = _CurrentEvent
        ),
        Table_Events[End]
    )

RETURN _EndOfSelectedEvent - _StartOfSelectedEvent

And then we can calculate duration of each of subevents:

Subevent Duration = 
VAR _CurrentEvent = SELECTEDVALUE( Table_Events[Event] )
RETURN
    IF(
        Table_Events[Subevent] <> "Idle",
        Table_Events[End] - Table_Events[Start],
        Table_Events[Duration of Event] - 
            SUMX(
                FILTER(
                    Table_Events,
                    Table_Events[Event] = _CurrentEvent
                ),
                Table_Events[End] - Table_Events[Start]
            )
    )

I've uploaded a sample *.pbix file here. I'll keep it here for you for the next 20-30 days 🙂