title: deck.gl transitions and filtering
body: <p>Can anyone explain how data updates and transitions work with Deck.gl compared to d3js? For instance in this code: </p>
  
  <pre><code>var updateLayers = function(dataset) {
  var scatterplot = new deck.ScatterplotLayer({
      /* unique id of this layer */
      id: 'checkins',
      /* data: an array of objects */
      data: dataset,
      /* data accessors */
      getPosition: d =&gt; d.geometry.coordinates, // returns longitude, latitude, [altitude]
      getRadius: d =&gt; circleSize(d.properties.reviews), // returns radius in meters
      getColor: d =&gt; [255, 0, 0],
      outline: true, // returns R, G, B, [A] in 0-255 range
      transitions: {
          getRadius: {
              duration: 1000,
              easing: d3.easeCubicInOut,
              enter: value =&gt; [value[0], value[1], value[2], 1] // fade in
          }
      }
  })
  
  // Add the layer to deckgl:
  deckgl.setProps({ layers: [scatterplot] });
  }
  </code></pre>
  
  <p>It's unclear to me what <code>enter: value =&gt; [value[0], value[1], value[2], 1]</code> is doing. Can anyone explain? I would generally expect (per d3js) that this <code>enter:</code> is setting the breakpoints in the transition, but I'm unclear what <code>value</code> refers to?</p>