title: adding reference line in line chart based on a column
body: <p>i have a sample of  data frame like this</p>
  <p><a href="https://i.stack.imgur.com/VL2ZV.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/VL2ZV.png" alt="pic1" /></a></p>
  <p>i want to plot a line chart with index on x axis and amplitude on y axis for that I am using</p>
  <p><a href="https://i.stack.imgur.com/YDJDN.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/YDJDN.png" alt="pic2" /></a>
  I got a line chart but how can I add reference line to show that a quake happened at a point I am expecting a chart like this or similar</p>
  <p><a href="https://i.stack.imgur.com/1OcKD.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/1OcKD.png" alt="pic" /></a></p>