title: Show "loading graph" message in plotly
body: <p>I want to show message like "loading graph" while plotting is in progress.</p>
  
  <p>How do I achieve this?</p>
  
  <p>Expected output:<br>
  <a href="https://i.stack.imgur.com/3hyci.png" rel="nofollow"><img src="https://i.stack.imgur.com/3hyci.png" alt="enter image description here"></a></p>
  
  <p><a href="https://i.stack.imgur.com/ENhYW.png" rel="nofollow"><img src="https://i.stack.imgur.com/ENhYW.png" alt="enter image description here"></a></p>