15. The scene, and the way out

Everything in this guide compiles to one scene, a tree of ten kinds of node, and every backend draws only that: Path, Text, Markers, Raster, Group, Transform, Clip, Opacity, Keyed and Annotated. It is the level at which a figure that came out wrong can be read.

let fallScene = Chart.toScene size fall
printfn "the dropped ball is %d nodes and %d primitives" (Scene.count fallScene) (Scene.primitiveCount fallScene)

A scatter of any size is one Markers node, which is what lets the GPU draw a million points as one instanced call. Keyed and Annotated carry identity and meaning through to the output: SVG gets ids and ARIA roles, TikZ gets comments, so a rendered figure can be diffed and read as text. A figure names its parts figure, figure/panel-0-0, figure/panel-0-0/layer-0, spelled by Keys, and Scene.mapKey rewrites one part by name and leaves every other part as it was:

let faded = fallScene |> Scene.mapKey (Keys.layer (Keys.panel 0 0) 0) (Scene.opacity 0.25)
printfn "the model curve found and faded: %b" (Scene.tryFindKey (Keys.layer (Keys.panel 0 0) 0) faded).IsSome

Scene.opacity fades a group as one, which differs from setting each shape's alpha wherever the group's shapes overlap.

The way out

Your own scales. Chart.series builds a typed series against scales you state, and Chart.ofSeries draws it, so everything downstream works on scales of your choosing.

Erasing on purpose. Erase is the one place a float<'u> becomes a number with a runtime Dim. Below it there are no measures, which is how one backend serves every unit.

let statedScale = Scale.log10 U.metre (1.0<m>, 1000.0<m>)
let ownSeries = Chart.seriesIn "measured" U.second U.metre tModel xModel
let ownFigure = Chart.ofSeries GeomLine ownSeries

printfn "an erased scale carries %s" (Dim.show (Erase.scale statedScale).Dim)

Your own backend. A renderer is a function Extent<px> -> Node -> string, or bytes, and Chart.render takes any such function, with nothing to register:

let countingRenderer (_: Extent<px>) (node: Node) = sprintf "%d nodes" (Scene.count node)

printfn "%s" (fall |> Chart.render countingRenderer (Chart.size 100.0 100.0))

A backend that keeps state implements IBackend, a begin, draw and finish over a surface, and Chart.renderTo drives it. What a backend can do is its Capabilities, and what it cannot is degraded in a stated way: the SVG backend estimates text widths where Skia shapes them.

What Marey does not do yet is on its own page, Not in 1.0, and what a version number promises is on Compatibility.