Expression.md

+Mapnik Expressions enable data-driven behavior in Mapnik. They are constructed from strings, can hold any kind of type of data, and are evaluated at render time. They are used by symbolizers like the [[TextSymbolizer]] to dynamically construct text labels for display and they are also the basis for how Mapnik [Filters](Filter) work.
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+Recently, in Mapnik 2.1, expressions became enabled in the `transform` parser used to apply affine transformations to image and svg symbols. So, now you can not only, for example, rotate symbols with a transform like `transform="rotate(45)"` but the rotation can be data driven like `transform="rotate([field])"`
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+For more info on the original design see: http://mapnik.org/news/2009/12/08/future_mapnik2/
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+in the feature,I think the color attribute such as the stroke of 
+LineSymbolizer, the fill of PolygonSymbolizer, the fill of TextSymbolizer and so on, should also support Expression, this is more useful.
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