mapnik/tests/visual_tests/styles/functional-expressions.xml

<Map background-color="#eee" srs="+proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs">
    <Parameters>
        <Parameter name="sizes">256,256</Parameter>
    </Parameters>

    <Style name="frame" filter-mode="first">
        <Rule>
            <Filter>[mapnik::geometry_type]=linestring</Filter>
            <LineSymbolizer
                stroke="[color]"
                stroke-width="[width]"
                offset="abs([offset]*2.0)"/>
            <LineSymbolizer
                stroke="[color]"
                stroke-width="[width]"
                offset="-[offset]*2.0"
                geometry-transform="rotate(360-(tan(-180)*10))"/>
        </Rule>
        <Rule>
            <Filter>[mapnik::geometry_type]=point</Filter>
            <MarkersSymbolizer
                file="../../../[base-path]/[filetype]/[filename].[filetype]"
                width="exp([width])"
            />
        </Rule>
    </Style>

    <Layer name="frame" srs="+proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs">
        <StyleName>frame</StyleName>
        <Datasource>
            <Parameter name="type">ogr</Parameter>
            <Parameter name="extent">-10,-10,10,10</Parameter>
            <Parameter name="layer_by_index">0</Parameter>
            <Parameter name="inline"><![CDATA[
                {
                   "type" : "FeatureCollection",
                   "features" : [
                      {
                         "properties" : {
                            "width":"5",
                            "color" : "red",
                            "offset": 10
                         },
                         "geometry" : {
                            "coordinates" : [[-10,-10],[10,10]],
                            "type" : "LineString"
                         },
                         "type" : "Feature",
                      },
                      {
                         "geometry" : {
                            "coordinates" : [ 0,0 ],
                            "type" : "Point"
                         },
                         "properties" : {
                            "base-path":"./tests/data",
                            "filetype":"svg",
                            "filename":"octocat",
                            "width":"5",
                            "color" : "#ace"
                         },
                         "type" : "Feature",
                      }
                   ]
                }]]></Parameter>
        </Datasource>
    </Layer>
</Map>