title: mapnik markers dont always appear
body: <p>I'm using mapnik 2.2.0 from Python on Ubuntu 16.04.  I load OSM data and a layer of points from a csv file.  The points in the csv file are in epsg:4326.</p>
  
  <p>Different points appear or disappear depending on the size of the map.  The points that don't appear (depending on the map size) are in between points that appear.</p>
  
  <p>For example, in the 1024x1024 map, points 1, 5, and 8 from the list below do not appear, but they do appear in the 640x480 map.</p>
  
  <p>My code and sample points are below.  </p>
  
  <p>Any help is appreciated.</p>
  
  <p>Thanks.</p>
  
  <p>dl  </p>
  
  <p>points.csv</p>
  
  <p>longitude,latitude</p>
  
  <p>-81.36267786,31.17942128<br>
  -81.34324795,31.27446072<br>
  -81.47343063,31.24168062<br>
  -81.55010796,30.34051442<br>
  -81.80767273,30.31021431<br>
  -81.37953944,30.19135471<br>
  -81.7932574,30.19754696<br>
  -81.76737595,30.13418484<br>
  -81.84996796,30.08385658<br>
  -81.58109463,30.50306859<br>
  -81.49115618,30.51390896<br>
  -81.5587192,30.63135991<br>
  -81.57580261,31.25126381<br>
  -81.29788475,29.8236434  </p>
  
  <pre><code>#!/usr/bin/env python2
  
  import mapnik
  
  
  mapfile = '/etc/mapnik-v2.2.0/config/mapnik_osm.xml'
  
  #m = mapnik.Map (640, 480)
  </code></pre>
  
  <p>#m = mapnik.Map (1280, 960)
      m = mapnik.Map (1024, 1024)</p>
  
  <pre><code>m.background = mapnik.Color ('rgb (0,0,0,0)')
  m.aspect_fix_mode = mapnik.aspect_fix_mode.GROW_CANVAS
  
  mapnik.load_map (m, mapfile)
  
  style = mapnik.Style()
  rule1 = mapnik.Rule()
  marker_symbolizer = mapnik.MarkersSymbolizer()
  marker_symbolizer.allow_overlap = False
  marker_symbolizer.opacity = 1.0
  rule1.symbols.append(marker_symbolizer)
  style.rules.append(rule1)
  
  m.append_style('GPS_tracking_points', style)
  layer = mapnik.Layer('coordinates', '+proj=latlong +datum=WGS84')
  layer.datasource = mapnik.CSV(file="points.csv", layer_by_index=0)
  
  layer.styles.append('GPS_tracking_points')
  m.layers.append(layer)
  
  west, south, east, north = -82.299335015,29.8236434,-80.848517695,31.27446072
  bbox = (mapnik.Box2d (west, south, east, north))
  
  merc = mapnik.Projection('+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 +no_defs +over')
  longlat = mapnik.Projection('+proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs')
  
  transform = mapnik.ProjTransform(longlat,merc)
  merc_bbox = transform.forward(bbox)
  
  
  m.zoom_to_box(merc_bbox)
  
  mapnik.render_to_file(m, "map_example.png")
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/DVnCL.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/DVnCL.png" alt="640x480"></a>
  <a href="https://i.stack.imgur.com/dwzmh.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/dwzmh.png" alt="1024x1024"></a></p>