mapnik/tests/visual_tests/styles/shield-on-line-and-avoid-edges.xml

<Map>
  <Parameters>
    <Parameter name="sizes">512,512</Parameter>
  </Parameters>
<Style name="line" filter-mode="first" >
  <Rule>
    <LineSymbolizer stroke="#000000" />
  </Rule>
</Style>
<Style name="point-placement" filter-mode="first" >
  <Rule>
    <ShieldSymbolizer
      allow-overlap="true"
      face-name="DejaVu Sans Book"
      file="shape://ellipse"
      placement="line"
      spacing="30"
      transform="scale(4,10)"
      fill="blue"
      avoid-edges="true"
      minimum-padding="5"
      clip="false">
    </ShieldSymbolizer>
    <DebugSymbolizer />
  </Rule>
</Style>
<Layer name="carto_tests">
    <StyleName>line</StyleName>
    <StyleName>point-placement</StyleName>
    <Datasource>
       <Parameter name="type">csv</Parameter>
       <Parameter name="file">../data/marker-on-line.csv</Parameter>
       <Parameter name="separator">|</Parameter>
    </Datasource>
  </Layer>
</Map>