Reference Line in the Y-axes is not displayed?

[qlikconsultant]

I have the followind value in the load script:

time(floor(bettime,1/(24*60)),'hh:mm') as mytime

In a bar chart i want for example a reference line at '21:40'

So I write in the expression ='21:40'

And choose at the location Secondrary Y.

But nothing is displayed.

Where is the error?