Reference line with custom value (HH:SS:MM)

[Cupcakes]

Hi, 

I would like to add two reference lines in my graph with a custom value to set a target and ambition like so:

Above is the old graph with the times as seconds. 

After a meeting with my team the stakeholder would rather have human readable times in their graph like so:

measure: average_talk_time {
    type: number
    value_format: "HH:MM:SS"
    sql: round((sum(${t_talk_acd})-sum(${t_hold_acd})) /sum(${n_answered_acd}))/86400 ;;
  }

This is the code used to calculated and format the seconds to a timestamp.
Only the last step is missing, the reference lines.

When I now add a reference line with its default setting:
Type: Line
Value: Average (Mean) 
The line will be shown in the graph as expected. 

Now let's try to set this to a custom value: 
Type: Line
Value: Custom… 
Value input: 370
Value format: HH:MM:SS
Result: Nothing happens / no line

With:
Type: Line
Value: Custom… 
Value input:  00:06:10
Value format: HH:MM:SS
Result: Nothing happens / no line

Is it possible to have a custom reference line when the measurement is time based?  Thank you in advance 🙂