title: Plotly: How to add markers at specific points in plotly line graph (python / pandas)
body: <p>I have a df (below) and want to write a function that will return a <code>plotly</code> line graph with data labels at specified points within the dataset (this graph tracks the success of a promotion, so I want to mark when the promo begins and ends - but that is not the same as the start and end of the dataset itself).</p>
  <pre><code>index   date    x_sales y_sales
  0   2019-10-24  0   27
  1   2019-10-25  0   30
  2   2019-10-26  0   34
  3   2019-10-27  0   36
  4   2019-10-28  0   29
  </code></pre>
  <p>The <code>plotly</code> docs are unclear about how to add annotations and the only relevant StackOverflow question I can find is about R.</p>
  <p>Ideally, I would like a graph like the one here:</p>
  <p><a href="https://i.stack.imgur.com/HvQ6v.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/HvQ6v.png" alt="sales graph with x_sales and y_sales" /></a></p>
  <p>but with a little label for <code>start date</code> and one for <code>end date</code> on one of the lines.</p>
  <p>My current code is as basic as it gets:</p>
  <pre><code>fig = px.line(df_g, x=&quot;date&quot;, y=df_g.columns)
  </code></pre>
  <p>Thanks for any help.</p>