Re-implement the visualization from https://github.com/shawnbot/concoct in one fell swoop using Mapnik 2.1

re-concoct.xml

<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE Map [

<!-- compositing mode: https://github.com/mapnik/mapnik/blob/master/include/mapnik/image_compositing.hpp#L42-79 -->
<!ENTITY comp_op "darken">

<!-- leave blank to render all data or set to some integer to render faster -->
<!ENTITY row_limit "">

<!-- size of marker ellipse -->
<!ENTITY point_size "7">

]>
<Map
  srs="+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs"
  background-color="white"
  minimum-version="2.1.0"
>
  <!--
    trees,crimes,cabs: rgb/cmy(k)
    https://github.com/shawnbot/concoct
    git clone https://github.com/shawnbot/concoct
    cd concoct
    nik2img.py -f png -d 1000 1000 -c -122.43 37.758 -z 13 --bbox-factor 1.5 re-concoct.xml mapnik.png
  -->

  <Style name="trees" comp-op="&comp_op;">
    <Rule>
      <MarkersSymbolizer fill="cyan" width="&point_size;" stroke-width="0" allow-overlap="true" />
    </Rule>
  </Style>

  <Style name="crimes" comp-op="&comp_op;">
    <Rule>
      <MarkersSymbolizer fill="magenta" width="&point_size;" stroke-width="0" allow-overlap="true" />
    </Rule>
  </Style>

  <Style name="cabs" comp-op="&comp_op;">
    <Rule>
      <MarkersSymbolizer fill="yellow" width="&point_size;" stroke-width="0" allow-overlap="true" />
    </Rule>
  </Style>

  <Datasource name="csv-data">
    <Parameter name="type">csv</Parameter>
    <Parameter name="base">./data/</Parameter>
    <Parameter name="row_limit">&row_limit;</Parameter>
  </Datasource>

  <Layer name="trees" srs="+proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs">
    <StyleName>trees</StyleName>
    <Datasource base="csv-data">
      <Parameter name="file">trees.csv</Parameter>
    </Datasource>
  </Layer>

  <Layer name="crimes" srs="+proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs">
    <StyleName>crimes</StyleName>
    <Datasource base="csv-data">
      <Parameter name="file">crimes.csv</Parameter>
    </Datasource>
  </Layer>

  <Layer name="cabs" srs="+proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs">
    <StyleName>cabs</StyleName>
    <Datasource base="csv-data">
      <Parameter name="file">cabs.csv</Parameter>
    </Datasource>
  </Layer>

</Map>