Legending
Should be configured with XML or a python class.
#536 tracks progress on this feature.
<Legend>
<Title breakline=true stroke-width="1" stroke="black">Education</title>
<SubTitle> Level of Education Among Population Over Age 20</Subtitle>
<Item>
<Symbolizer>
<CssParameter name="stroke">white</CssParameter>
<CssParameter name="stroke-width">.5</CssParameter>
<CssParameter name="fill">#93A462</CssParameter>
<ItemText face_name="DejaVu Sans Bold" size="10" fill="black">Highest</ItemText>
</Item>
....
</Legend>Default behavior would will be to use, for example, a filled rectangle to represent polygon geometries (like the example graphic above)
But an optional filter could be applied to direct the legend renderer to use a specific feature geometry (scaled to fit the legend layout) as the symbol
That would look something like:
Syntax might look like:
<Item>
<Filter>[id] = 1</Filter>
<Symbolizer>
...
# or, more simplistically choose a random feature
<Item use_feature="true">
<Symbolizer>
...I'd like to approach things slightly differently. We could keep the stylesheet as close as possible to the "real" one, and concentrate the legend-specific alterations to the layer definitions. For example, if there was a WKT layer (#630) and screen coordinates (#631) then you could have something like
[... normal style rules ...]
<Layer name=pubs srs="screen">
<Datasource type="wkt">
<Parameter name="geom">POINT(20,20)</Parameter>
<Parameter name="attributes">{"amenity","pub"}</Parameter>
</Datasource>
</Layer>The label text for the legend could then be chosen and placed with other screen+wkt layers to get the desired effect. Although this means creating custom layers, it's likely to be less work than altering the <style> sections of a large map definition.
Lars Ahlzen has written a Python script for TopOSM which creates HTML snippets with images from a Mapnik style file: http://wiki.openstreetmap.org/wiki/TopOSM/Details#Map_legend