title: How can I add a ruler (vertical line) to CodeMirror editors in JupyterLab?
body: <p>I would like to add a ruler at 80 characters. I know that you can <a href="https://codemirror.net/demo/rulers.html" rel="noreferrer">add rulers to CodeMirror</a>, but I don't know if I'm supposed to place <a href="https://codemirror.net/addon/display/rulers.js" rel="noreferrer">the corresponding javascript</a> in a file somewhere, or in JupyterLab's <code>Advanced Settings Editor</code> > <code>CodeMirror</code> > <code>User Overides</code> section, or if I should do something else all together to get it working.</p>
  
  <p>I'm using JupyterLab 0.31.8</p>