title: interactive map with leafletR variable point size
body: <p>I create an interactive map as follows:</p>
  
  <pre><code>library(leafletR)
  data(quakes)
  
  # store data in GeoJSON file (just a subset here)
  q.dat &lt;- toGeoJSON(data=quakes[1:99,], dest=tempdir(), name="quakes")
  
  # make style based on quake magnitude
  q.style &lt;- styleGrad(prop="mag", breaks=seq(4, 6.5, by=0.5), style.val=rev(heat.colors(5)), leg="Richter Magnitude", fill.alpha=0.7, rad=8)
  
  # create map
  q.map &lt;- leaflet(data=q.dat, dest=tempdir(), title="Fiji Earthquakes", base.map="osm", style=q.style, popup="mag")
  
  # view map in browser
  rstudio::viewer(q.map)
  </code></pre>
  
  <p>Now, I want to make the size of the circle dependent on another variable. Let's say the variable 'stations'. How can I do this? If it is not possible with this package, I am open to use another package ... as long as I can put a legend, the map is interactive, a pop-up appear when clicked on and the color can depend on the value of a continuous variable.</p>