HLine

Property	Default	Meaning
y	0.5	Fixed y coordinate
lim	[0, 1]	Start and end x coordinates
space	Automatic	Use ambient data coordinates when available; otherwise local geometry

HLine draws from [lim[0], y] to [lim[1], y]. Outside a coordinate context, the defaults span the width of its own allocated rectangle at half its height. Position and span accept lengths: fractions, px, em, and unit strings such as "25%" or "12px". Reversed and equal span endpoints are allowed. Geometry does not change the element's layout size.

Inside Graph, Plot, or GeoMap, numeric endpoints use ambient coordinates automatically and contribute to ordinary Graph/Plot limit inference. Set space="local" for a local rule, or space="data" to require a coordinate context. Defaults remain [0, 1] for lim and 0.5 for the fixed position; they do not expand to the graph limits. Tagged lengths retain Line's local-length behavior. Custom projections map the two endpoints, so the resulting segment need not be horizontal on screen.

from and to are not supported; use Line for arbitrary endpoints. Use VLine for a fixed x coordinate and a span along y.