# Custom Marks and Renderers Use a custom mark when a visualization fits the shared scene model but is not expressible as a useful composition of built-in Cartesian, polar, or geographic marks. Use a custom renderer when the same chart scene needs a different mounted surface. Use a custom SVG serializer when only SVG markup or resources differ. Neither extension should reach into private scene compiler state. ## Start with composition Before creating a mark, check whether the result is a combination of: - lines or areas; - rectangles or cells; - dots or hexagons; - rules, links, ticks, arrows, or vectors; - text or frames; - facets. - polar arcs, radial paths, dots, or guides; - projected GeoJSON. Composition retains built-in type inference, focus metadata, animation, and subpath bundle boundaries. The [chart examples](../examples/index.md) show boxplots, candlesticks, networks, and annotations built this way. ## Create a mark `createMark` is the normal extension boundary: ```ts import { createMark } from '@tanstack/charts' interface ThresholdDatum { id: string value: number } const threshold = createMark(({ markIndex }) => { const id = `threshold-${markIndex}` const datum: ThresholdDatum = { id: 'target', value: 75 } return { id, channels: { y: { scale: 'y', values: [datum.value], }, }, render({ chart, scales, theme }) { const y = scales.y.map(datum.value) return { nodes: [ { kind: 'rule', key: datum.id, x1: chart.x, x2: chart.x + chart.width, y1: y, y2: y, style: { stroke: theme.foreground, strokeOpacity: 0.55, }, }, ], } }, } }) ``` `initialize` materializes channels for one scene build. `render` receives the required full `surface` bounds, inner `chart` plot bounds, scales, theme, color resolver, and text layout tools. When a custom mark emits data labels, an optional `layoutLabels(context)` can return those positioned `SceneLabel` nodes before render so unlocked margins contain them. Keep that method pure because responsive layout may call it more than once; the final `render` call still happens once. Available scene nodes: - `group`; - `rule`; - `polyline`; - `area`; - `dot`; - `rect`; - `label`. Every node requires a deterministic key. ## Interaction points The threshold above is decorative, so it emits no points. Return `ChartPoint` records when custom geometry should participate in focus, tooltips, keyboard navigation, or selection. Each point should retain: - its original datum; - a stable key; - semantic x and y values; - resolved pixel coordinates; - group identity and color. For a large painted mark, create the semantic point once and attach that same object to the scene primitive that paints it: ```ts import type { SceneRect } from '@tanstack/charts' const point = interactionPoint(index) const node: SceneRect = { kind: 'rect', key: point.key, x, y, width, height, interaction: { point, affinity: 'x' }, } return { nodes: [node], points: [point] } ``` Use `x` for vertically oriented marks, `y` for horizontal marks, `xy` for ordinary two-dimensional proximity, and `geometry` when only exact containment should focus the mark. The default resolver checks containment across every mark before applying any fallback. A continuous `polyline` or `area` may attach all of the semantic samples it represents with `interaction: { points, affinity }`; containment selects the closest sample within that primitive. Keep primitive coordinates local when returning translated groups. Scene traversal applies nested translation, clipping, facets, and paint order after layout. Do not calculate a second set of global hit bounds beside the rendered node. Omit points for decorative geometry. Do not invent fake interactive data for a frame, grid, or threshold that should not receive focus. ## Focus-only anchors A decorative mark can still support `whenFocused` without becoming a pointer target. Return `focusAnchors` beside its nodes: ```ts return { nodes: [node], focusAnchors: [ { key: node.key, markId: id, group: null, datum, datumIndex: index, yValue: datum.value, }, ], } ``` The anchor key must identify the node or keyed group it reveals. Include only the semantic axes the geometry owns: a horizontal rule supplies `yValue`; a vertical rule supplies `xValue`. `focusAnchors` are read only when the mark is wrapped in `whenFocused` and never enter pointer hit testing, tooltip data, or keyboard navigation. ## Focus-guide marks A mark that emits only cursor-driven rules, bands, labels, or markers declares that role explicitly: ```ts import { resolveCrosshairGuide } from '@tanstack/charts/crosshair' return { id, channels: {}, focusGuideOnly: true, render({ chart, surface, scales, theme }) { return { nodes: [], focusGuides: [ { key: id, markId: id, chart, surface, x: { style: { stroke: theme.foreground } }, projectX: (value) => { const scale = scales.x if (!scale || scale.type === 'none') return undefined const position = (scale.viewport?.map ?? scale.map)(value) return Number.isFinite(position) ? position : undefined }, resolve: resolveCrosshairGuide, }, ], } }, } ``` `focusGuideOnly: true` keeps a guide-only mark from becoming the first ordinary mark used to divide underlays from overlays. `MarkScene.focusGuides` accepts `MarkFocusGuide`. Its `placement` is optional: omit it for normal mark-order placement. An explicit `under` or `over` is reserved for composed nested scenes that must retain placement already resolved inside that composition. Authors do not need to invent a placement for an ordinary guide mark. The required `surface` bounds cover the complete chart surface; `chart` covers the inner plot. Use `chart` for clipped rules and `surface` for labels that must remain visible. Project semantic guide values through `scale.viewport?.map ?? scale.map` so a transient viewport translation keeps the guide aligned with presented content. Each guide's required `resolve` callback receives the final guide, local focus, pointer, and projected cursor, then returns one transient scene node or `undefined`. `resolveCrosshairGuide` provides the built-in rule, band, label, and marker behavior. A custom guide can supply different policy without adding it to renderer bundles that never use the guide. A custom renderer receives final `SceneFocusGuide` values after the compiler has filled in placement. Pass the scene to `resolveFocusPresentation` instead of calling guide resolvers or resolving mark order inside the renderer. ## Separate point and scale values Most marks use the same value type for interaction and scale domains. When they intentionally differ, import the advanced factory: ```ts import { createMarkWithScaleValues } from '@tanstack/charts/mark/scale-values' createMarkWithScaleValues(initialize) ``` This is useful for interval endpoints or custom layouts whose interactive anchor is not the complete set of values materialized on an axis. Use `ChartMarkPointX` and `ChartMarkPointY` for the interaction contract and `ChartMarkScaleX` and `ChartMarkScaleY` for the positional contract. Ordinary chart code should rely on definition inference instead. ## Custom scales and legends Configured callable scales are the normal path. `ChartScale`, `ChartColorScale`, and `ChartColorLegend` exist for context-aware adapters that need chart range, theme, or responsive legend geometry. Keep specialized scale dependencies in the module that uses them. A line-only bundle must not pay for a custom scale registered elsewhere. See [Scales](../concepts/scales-and-d3.md) and [Legends and Color](./legends-and-color.md). ## Custom renderer A full renderer implements `ChartRenderer` and returns a `ChartSurface`: ```ts import { mountChartRenderer } from '@tanstack/charts/renderer' const host = mountChartRenderer(container, { definition, renderer: myRenderer, ariaLabel: 'Threshold history', }) ``` The renderer owns server shell markup, its mounted element, scene painting, focus painting, and cleanup. It can implement `clientToScene` when controlled pointer gestures need client-coordinate conversion; the interaction controller returns `null` when that optional capability is absent. The host retains sizing, runtime, keyboard, tooltip, selection, and focus-strategy behavior. Keep `prerender` deterministic and make `mount` adopt compatible server markup. If `paintFocus` resolves and paints inline mark-state geometry, return that destination `ChartScene`. The host will use it for subsequent pointer hits; returning nothing preserves base-scene interaction for simpler renderers. Call `resolveFocusPresentation(scene, focus, pointer, cursor)` to obtain the authored and crosshair nodes for the renderer's underlay and overlay surfaces. If the renderer animates point geometry, implement `getPresentationPoints` and `subscribePresentationPoints`. This keeps stationary pointer focus, keyboard focus, and tooltip anchors aligned with the painted frame. Use `ChartRendererRenderContext.surface` instead of assuming `onRender` exposes an SVG element. Framework consumers pass `renderer` through `@tanstack/react-charts/core` or `@tanstack/octane-charts/core`. ## Custom SVG serializer A `ChartSvgRenderer` accepts the complete `ChartScene` and accessible SVG options: ```ts const renderSvg: ChartSvgRenderer = (scene, options) => { return serializeMySvg(scene, options) } ``` Pass it through `renderSvg` on the vanilla host or any default SVG framework adapter. Preserve: - the accessible label and description; - stable `data-ts-key` identity when DOM reconciliation should reuse nodes; - the focus marker contract when native focus remains enabled; - scoped IDs through `idPrefix`; - deterministic server output. The default `renderChartSvg` already emits declared gradients and group clips. The compatible `renderChartSvgWithResources` export remains available when an explicit resource serializer name is useful. Mounted SVG surfaces also call the selected serializer when focus guides are painted. That call contains a single `focus-guide-layer:under` or `focus-guide-layer:over` group in `scene.nodes`; preserve its keyed `` and apply the same paint, clipping, and resource-ID rules as the base scene. ## Custom focus and spatial indexes A `ChartFocusStrategy` owns pointer resolution, grouping, and keyboard navigation. Pointer coordinates and the point being grouped arrive through the second context bag. Its generic types must remain identical to the chart points it receives. A `ChartSpatialIndexFactory` builds optional nearest-point acceleration from scene points and receives the complete resolved scene through `context.scene`. Return original typed points from the index. Do not erase them to `unknown` and cast them back in callbacks. ## Extension checklist - Built-in composition was considered first. - Channel values fully declare positional domain inputs. - Scene generation is deterministic and DOM-free. - Keys survive reorder and updates. - Interactive points retain original data and exact coordinate types. - Decorative geometry emits no fake points. - Optional dependencies remain behind the extension import. - Custom rendering preserves accessibility, identity, and SSR. - No cast, private import, or suppression is needed at the public boundary.