Some of the output produced by our team - primarily on updates for 'legacy' projects - is styled with diagonal line fills (an example appears below).
In mapnik, this can be achieved using
<PolygonPatternSymbolizer file="[appropriate image]"/>
The exact same effect is possible "image-free" using
background-image in CSS3, styled with appropriate gradients.
My preference is for the latter.
My searches indicate that this is not possible in mapnik i.e. it appears that mapnik's PolygonSymbolizer has parameters (fill, fill-opacity, gamma); PolygonPatternSymbolizer takes (file).
I do notice that PolygonSymbolizer.fill 'accepts expressions', as does PolygonPatternSymbolizer.file - so the question is... can expressions be used to achieve the same effect as the image above?