title: How to plot the function definition domain
body: <p>I would like to plot in a figure the domain my function is defined like the following <img src="https://i.stack.imgur.com/O4me8.png" alt="enter image description here"></p>
  
  <p>Is there a nice way to do it within R?</p>
  
  <p><strong>UPDATE:</strong> I do not want to plot the function I want to plot the <strong>domain description</strong> (as a <code>legend</code> or <code>text</code>) like the image below <img src="https://i.stack.imgur.com/gpSgp.png" alt="enter image description here"></p>