title: Curved `geom_line()` using group aesthetic
body: <p>I have a data that is of a "grid" structure and I wish to connect certain points together according to a group variable. <code>geom_line()</code> can achieve this (see below), but I want to get those lines to be curved. How could this be done? <code>geom_curve()</code> does not seem compatible with this particular data format. Thank you.</p>
  
  <pre><code>df = data.frame(
   x = factor(c(1,1,2,2)),
   y = factor(c(1,2,1,2)),
   group = c("A","A","B","B")) 
  
  df %&gt;% 
   ggplot(aes(x = x, y = y)) +
   geom_point() +
   geom_line(aes(group = group))
  </code></pre>