title: Only plotting the fitted spline line and not the data points
body: <p>I have checked my references, it seems to me that to fit a dataset with x and y, many tutorial need to first plot the x and y,  then the fitted line is plot. The normal procedure is like below:</p>
  
  <pre><code>## Calculate the fitted line
  smoothingSpline = smooth.spline(tree_number[2:100], jaccard[1:99], spar=0.35) 
  plot(tree_number[2:100],jaccard[1:99]) #plot the data points
  lines(smoothingSpline) # add the fitted spline line.
  </code></pre>
  
  <p>However, I do not want to plot the tree_number and jaccard, but rather, I only want to plot the fitted spline line in the plot, how should I do? </p>