How do I assign and display a threshold or trendline on a chart? For
example, I would like to show an established and pre-determined threshold
line to show across my bar chart to show if the data lies above or below that
mark. So, it would be a straight line across rather than declining or
inclining like a trendline does.

I have never worked with one and I understand how the trendline can work,
but do not know how to display something like this. Is this possible?