title: why geom_smooth not showing the smooth line?
body: <p>I am having a problem where geom_smooth() is not working on my ggplot2. 
  But instead of a smooth curve, there is a fold.
  <img src="https://i.stack.imgur.com/V3dPq.png" alt="enter image description here">
  My X-axis variable is the factor variable(I've tried to convert it to a numerical variable, but it didn't work), and Y-axis is numeric variable.
  My data.frame is that <img src="https://i.stack.imgur.com/dtFBN.png" alt="enter image description here"> <img src="https://i.stack.imgur.com/4pJuY.png" alt="enter image description here"></p>
  
  <pre><code>ggplot(tmp, aes(x = x, y = y))+
    geom_point()+
    geom_smooth(formula = y ~ x, method = "loess", stat = "identity", se = T, group = "")
  </code></pre>
  
  <p>I hope to get a pic like this.
  <img src="https://i.stack.imgur.com/sjfuR.png" alt="enter image description here"></p>