title: Cannot convert Float64 to series data for plotting in Julia
body: <p>The following is the code I wrote.
  I have no idea why the error happened and how to solve it.
  <a href="https://i.stack.imgur.com/lAc6q.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/lAc6q.png" alt="enter image description here" /></a></p>