mapnik/tests/visual_tests/styles/expressionformat.xml

<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE Map>
<Map background-color="white" srs="+proj=latlong +datum=WGS84">

  <Layer name="layer" srs="+proj=latlong +datum=WGS84">
    <StyleName>My Style</StyleName>
    <Datasource>
      <Parameter name="type">shape</Parameter>
      <Parameter name="file">../data/points.shp</Parameter>
    </Datasource>
  </Layer>

    <Style name="My Style">
    <Rule>
      <PointSymbolizer/>
      <TextSymbolizer face-name="DejaVu Sans Book" placement="point" dx="0" dy="5">
          <ExpressionFormat size="[nr]">[name]</ExpressionFormat>
      </TextSymbolizer>
    </Rule>
  </Style>

</Map>