title: requiring a a fixed horizontal line on a times series plot
body: <p>i have a plotly plot based on a time series but now im required to add a fixed horizontal like at y=238 to it 
  can you please give me some ideas?</p>
  
  <pre><code>fig= plot_ly(
    data = x,
    x = x$time,
    y = x$infected,
    mode = 'lines'
  )
  
  fig
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/x52uG.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/x52uG.jpg" alt="enter image description here"></a></p>