Arbitrary fill patterns! For example geologists haveall sorts of standardized patterns they know and love, and this lets you create any pattern you want. It still has the constraint of two colors (foreground and background) and a square unit cell, but now any svg path string is accepted to define the foreground color area.

I chose to do this by adding a new attribute in thepattern container,path. We could instead put the path into theshape attribute - in fact we only accept one (shape takes precedence) and for drawing I combine them into one anyway. And this is what we do forline.dash for example, there's an enumerated set but you also have the option of providing an arbitrary string in the same place. On the other handlayout.shapes has apath attribute fortype='path' shapes and other attributes for other shapes. Alsoline.dash is somewhat awkward to describe (astring attribute that nevertheless listsvalues like an enum?) andshape='<path string>' is a bit funny to read. I don't feel strongly about this though - if others would prefer to add this into theshape attribute that would also be fine.