title: How to make dots in gganimate appear and not transition
body: <p>I am using gganimate. Say I have this MWE:</p>
  
  <pre><code>library(ggplot2)
  library(gganimate)
  ggplot(airquality, aes(Day, Temp)) +
      geom_point(color = 'red', size = 1) +
      transition_time(Month) +
      shadow_mark(colour = 'black', size = 0.75)
  </code></pre>
  
  <p>I have one question: how can I make the new points just appear as opposed to transitioning from the old ones? Put another way, I just want the new dots to appear at their final location and not have a transition. How should I modify the code?</p>