title: geom_line : How to connect only a few points
body: <p>I have this dataframe and this plot : </p>
  
  <pre><code>df &lt;- data.frame(Groupe = rep(c("A","B"),4),
                   Period = gl(4,2,8,c("t0","t1","t2","t3","t4")),
                   rate = c(0.83,0.96,0.75,0.93,0.67,0.82,0.65,0.73))
  
  ggplot(data = df, mapping = aes(y = rate, x = Period ,group = Groupe, colour=Groupe, shape=Groupe)) +
     geom_line(size=1.2) +
     geom_point(size=5) 
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/VQibl.png" rel="noreferrer"><img src="https://i.stack.imgur.com/VQibl.png" alt="Plot"></a></p>
  
  <p>How could i organize my data so that the points between <code>t1</code> and <code>t2</code> are not connected with a line ? I'd like <code>t0</code> and <code>t1</code> to be connected (blue or red according to the group), <code>t2</code> and <code>t3</code> connected in the same way, but no lines between <code>t1</code> and <code>t2</code>. I tried several things by looking at similar questions, but it always mess up my grouping colors :/</p>