The constant line feature in PowerBI seems to be one of those hidden little gems. Lets say you have an SLA with a customer of the amount of downtime allowed on the system and you want to show on a graph once they have gone above this downtime threshold, you can add a constant line to your chart.
In my example below I set the constant line to 1000 and anything above that is great for the purpose of this chart.
To set a constant line, plot your graph, then on the right hand side, select what looks like a magnify glass. This will only show if you have the graph selected.
Then your constant line shall appear on your graph in the colour and density you set it to.
Hope this little gem helps you out in the future!