[viz] breaking change: refactor label handling in viz specs, :format now unused and replaced with :label key taking a function emitting label elements instead of just formatting label values, add default-svg-label fn, re-format examples
[viz] breaking change: refactor label handling in viz specs, :format now
unused and replaced with :label key taking a function emitting label
elements instead of just formatting label values, add default-svg-label
fn, re-format examples

thi-ng/geom,thi-ng/geom