A pattern fill for an area that has no single colour to draw — today
just a scale’s na_pattern (#206): ScaleFillBinned::na_pattern,
drawn instead of na_value’s flat colour by geom_sf/geom_polygon/
geom_rect/geom_tile and the legend’s “no data” key, so a classed
choropleth’s no-data cells read as “we have no reading here” rather
than a greyed-out value easily mistaken for one.
Line cap style (R grid’s lineend), applied to the stroked ends of a
polyline/path. Butt is SVG’s own default, so it is written on the wire
only when it differs (byte-identical default output).