title: R ggplot2: continous scale based on index
body: <p>I have a large dataframe (94k+ values) with lat/lon values. Short example:</p>
  <pre><code>df &lt;- data.frame(lat = c(50, 60, 70, 80, 90), 
                   lon = c(25, 28, 30, 32, 35))
  </code></pre>
  <p>But because the original dataframe is so large, I want to fill in the colour of the plotted points (with geom_point) based on the index value. So it shows the path of the observations over time with a gradient.</p>
  <p>Current ggplot:</p>
  <pre><code>ggplot(data = df) +
    geom_point(aes(x = lon, y = lat)
  </code></pre>