I understand that you would like to include a threshold line on real time plots. If you are using a scope and want to add a line at a given value, you can pass a constant value in to the scope. You can just add an extra input to the scope block and use a constant block's output to plot that threshold
<https://www.mathworks.com/help/simulink/slref/scope.html>
I am attaching a file "plotThreshold.mdl" . The blocks on the left show how to add threshold lines. If you are using Simulink Real-Time, the commented out block on the right shows a simple version.
