EXERCISE 8: Finish Interactive Example from Class

//   Entering Step Zero
     d3.select("#step-zero").on('stepin',function(e) {
         d3.select("#yaleBuildings").classed('hidden', true)
         d3.select("#bubblePolygon").classed('hidden', true)
      })

      //   Entering Step One
      d3.select("#step-one").on('stepin',function(e) {
         d3.select("#yaleBuildings").classed('hidden', false)
      })

      //   Entering Step Two
      d3.select("#step-two").on('stepin',function(e) {
         d3.select("#bubblePolygon").classed('hidden', false)
      })

      //   Entering Step Three
     d3.select("#step-three").on('stepin',function(e) {
         d3.select("#yaleBuildings").classed('hidden', true)
         d3.select("#bubblePolygon").classed('hidden', true)
      })