Fan Chart API

Plot the forecast and its uncertainty in one chart: history, a dashed median, and confidence bands that widen with the horizon - see the Fan Chart demo.

Import

Web Component

import "@michi-vz/wc/fan-chart";
// <michi-vz-fan-chart> is now defined

Vanilla JS

import { mountFanChart } from "@michi-vz/core";

const chart = mountFanChart(el, props);

Insights helper

import { forecastFan } from "@michi-vz/insights/forecast";

const item = forecastFan(history, { horizon: 4, levels: [0.5, 0.8], level: 0.95 }, "Revenue");
const chart = mountFanChart(el, { dataSet: [item], xAxisDataType: "date_annual" });

Props

Prop	Type	Default	Description
dataSet*	FanDataItem[]	—	Array of forecast series, each a median line plus its nested confidence bands
title	string	—	Optional chart title rendered above the plot
yAxisDomain	[number, number]	—	Fixed [min, max] for the y-axis; when omitted the domain auto-fits the line plus the widest band extents
xAxisDataType	"date_annual" | "date_monthly" | "number"	"number"	How x values are parsed and formatted: yearly dates, monthly dates, or plain numbers
xAxisFormat	(d: number | string) => string	—	Formats an x tick value into its display label
yAxisFormat	(d: number | string) => string	—	Formats a y tick value into its display label
ticks	number	5	Approximate number of axis ticks to generate
tickValues	Array<number | Date>	—	Explicit tick values, overriding the generated ones
curve	"curveBumpX" | "curveLinear" | "curveMonotoneX"	—	Line interpolation: curveLinear, curveMonotoneX, or curveBumpX
fillOpacity	number	0.18	band fill opacity for the widest band (narrower bands scale up from here).
forecastZone	boolean	—	shade the forecast region (from the last solid point to the end). Default true.
showDataPoints	boolean	false	Draw a marker at each median data point (default false)
tooltipFormatter	(item: FanDataItem, lastPoint: DataPoint | null) => string	—	Custom HTML for the tooltip (DOMPurify-sanitized; may include `<a href>` links).
onHighlightItem	(labels: string[]) => void	—	Called when the hovered/highlighted label(s) change

Common props — shared by every chart (14)

Prop	Type	Default	Description
width	number	1000	Chart width in pixels
height	number	500	Chart height in pixels
margin	Margin	{ top: 50, right: 50, bottom: 50, left: 60 }	Inner margins (top/right/bottom/left, in px) reserved for axes, titles, and labels
colors	string[]	—	Categorical palette for series/labels without an explicit colour or colorsMapping entry
colorsMapping	Record<string, string>	—	Explicit label -> colour map; takes precedence over the palette and per-item colours
highlightItems	string[]	—	Labels to emphasise; all other marks dim
disabledItems	string[]	—	Labels to hide and exclude from scales/stacks
renderer	"svg" | "canvas"	"svg"	Render as inline SVG (default) or to a canvas (faster for large datasets); getContext() is identical either way
locale	string	—	BCP-47 locale used for number and date formatting
skipColorMappingDispatch	boolean	false	External-CSS mode: unmapped labels resolve to transparent and onColorMappingGenerated is not emitted, so mark colours come from your CSS via the data-label-safe contract
enableTransitions	boolean	true	Animate updates with CSS transitions (default true)
onColorMappingGenerated	(mapping: Record<string, string>) => void	—	Called with the resolved label -> colour map after the chart assigns colours
onChartDataProcessed	(context: ChartContext) => void	—	Called with the renderer-agnostic ChartContext whenever the data is (re)processed
onDataWarning	(warnings: DataWarning[]) => void	—	Called with any non-fatal data warnings (duplicate labels, non-finite values, gaps, ...)

Events

The web component dispatches these bubbling CustomEvents (the engine exposes the same via the on* callbacks in the table above):

Event	Detail	Fires when
michi-vz:highlight	string[]	hover highlight changes
michi-vz:colormapping	Record<string, string>	a color mapping is generated
michi-vz:dataprocessed	ChartContext	data is (re)processed
michi-vz:datawarning	DataWarning[]	input warnings are detected

getContext()

mountFanChart(el, props).getContext() returns a renderer-agnostic FanChartContext (per-series history/forecast counts, band levels, final uncertainty, plus a deterministic natural-language summary + an a11y table). See LLM context.

Source

Props are typed as FanChartProps in @michi-vz/core.