selectively changing d3 elements when Backbone model changes

gistfile1.coffee

# note that this function is called whenever a GeoPoint model changes
# Backbone will pass in geo_point, which refers to the changed model
# @geo_points refers to the entire collection of GeoPoint models

# update the bound data
geo_points = @geo_points.reject (gp) => gp.get('markedForDelete')

geoPointMarkers = d3.select('#geo-point-layer').selectAll("g")
                    .data geo_points, (d) =>
                      d.cid

###
in here is the enter() code
###

# if there is a changed GeoPoint, go in and modify it      
# note that cid refers to the model's local id
if geo_point
  changedGeoPointMarkers = geoPointMarkers.filter (d, i) =>
    d.cid == geo_point.cid
  # select
  changedGeoPointMarkers.selectAll('circle')
    .attr "r", (d) ->
      if d.get 'selected'
        masterRouter.geo_point_layer.geoPointSelectedRadius
      else
        masterRouter.geo_point_layer.geoPointDefaultRadius
    .classed "selected", (d) ->
      d.get 'selected'
  # move
  @reapplyTransform()