title: Mapnik 2.2 disappearing markers (MarkerSymbolizer)
body: <p>I'm using Mapnik v2.2.0 from Python 2.7 in an Ubuntu 16.04 environment.  The application generates maps with a set of MarkerSymbolizers at various coordinates.  In one map, when the MarkerSymbolizers are set to 25x25 pixels, one of the markers does not appear.  If the marker size is set to 15x15, all of the markers appear.  </p>
  
  <p>The markers are loaded from a csv file:</p>
  
  <pre><code>    layer = mapnik.Layer('GPS_tracking_points')
      layer.datasource = mapnik.CSV(file=leak_file_name, layer_by_index=0)
  </code></pre>
  
  <p>The styling is below:</p>
  
  <pre><code>            point_style = mapnik.Style()
              point_rule = mapnik.Rule()
              point_filter = mapnik.Filter ("[val] &gt; 10")
              point_rule.filter = point_filter
              point_point_symbolizer = mapnik.MarkersSymbolizer()
              point_point_symbolizer.width = mapnik.Expression ('25')
              point_point_symbolizer.height = mapnik.Expression ('25')
              point_point_symbolizer.fill = mapnik.Color ('#ff0000')
              point_point_symbolizer.allow_overlap = True
              point_point_symbolizer.opacity = 0.5 # semi-transparent
              point_rule.symbols.append(point_point_symbolizer)
              point_style.rules.append(point_rule)
              map_canvas.append_style('high', point_style)
  </code></pre>
  
  <p>Screenshots are below.</p>
  
  <p>Any help is appreciated.</p>
  
  <p>Thanks.</p>
  
  <p><a href="https://i.stack.imgur.com/TH86Z.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/TH86Z.jpg" alt="Image missing Marker"></a></p>
  
  <p><a href="https://i.stack.imgur.com/Sfrct.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/Sfrct.jpg" alt="Image with all markers"></a></p>