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<!-- Name: Legending -->
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<!-- Version: 10 -->
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<!-- Last-Modified: 2011/10/10 02:01:25 -->
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<!-- Author: Petr Dlouhy -->
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# Discussion of a Legending Spec for Mapnik
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Should be configured with XML or a python class.
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#536 tracks progress on this feature.
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# Possible Example
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[[/images/legend_example.png]]
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## XML
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```xml
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<Legend>
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<Title breakline=true stroke-width="1" stroke="black">Education</title>
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<SubTitle> Level of Education Among Population Over Age 20</Subtitle>
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<Item>
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<Symbolizer>
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<CssParameter name="stroke">white</CssParameter>
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<CssParameter name="stroke-width">.5</CssParameter>
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<CssParameter name="fill">#93A462</CssParameter>
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<ItemText face_name="DejaVu Sans Bold" size="10" fill="black">Highest</ItemText>
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</Item>
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....
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</Legend>
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```
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## Python
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# Feature Wishlist
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### Layer symbolization using representative feature geometry
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Default behavior would will be to use, for example, a filled rectangle to represent polygon geometries (like the example graphic above)
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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
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That would look something like:
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[[/images/legend_feature_example.png]]
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Syntax might look like:
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```xml
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<Item>
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<Filter>[id] = 1</Filter>
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<Symbolizer>
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...
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# or, more simplistically choose a random feature
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<Item use_feature="true">
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<Symbolizer>
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...
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```
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### Andy's alternative approach
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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
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```xml
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[... normal style rules ...]
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<Layer name=pubs srs="screen">
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<Datasource type="wkt">
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<Parameter name="geom">POINT(20,20)</Parameter>
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<Parameter name="attributes">{"amenity","pub"}</Parameter>
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</Datasource>
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</Layer>
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```
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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.
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# See also
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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 |
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\ No newline at end of file |
