Create a histogram that track number of hours worked

I’m trying to create a histogram using the Graph Plugin that would display range of hours worked per day (for the last 30 days).
Very similar to this Hourly Heatmap plugin:

Idea is to have hours worked as Y value and date as X value like above.

The problem I have is that the values in the database are stored as Timestamp and I get an error:

Value column must have numeric datatype, column: value type: time.Time value: 0000-01-01 11:58:59 +0000 UTC

Using query:

      first_online::date as time,
      first_online::time as value

The first_online column value looks like:
2021-03-02 11:58:59
The first_online::date looks like:
The first_online::time looks like:

How exactly do you make Grafana accept first_online::time as a “value

Fixed it by converting it to epoch seconds and then telling the graph to interpret the Y axis as Time > clock(s)

first_online::date as time,
extract(epoch from w.first_online::time) as value