Heatmap

The simple version

Cells in a grid, colored by value. The same structure with almost no styling: a good place to start your own.

How it’s built

A heatmap is a Plot inside a Plot. The outer Plot draws one slat per day. Inside each day’s slat sits another Plot, <Plot orientation="across" v={d.hours}>, which draws one slat per hour. orientation="across" turns that inner Plot the other way, so its hours run along the day’s band instead of stacking, and its data comes from the day it sits in: d.hours.

• A Cell colors itself. It reads its value on the Chart’s scale and mixes the theme’s low and high colors by it, so there is no color scale to write. The gallery version’s theme sets low to a dark brown and high to amber, so hours with few visitors are close to the dark poster and busy hours are bright. ticks={false} on the Chart leaves the axis out.
• One slat draws the hour numbers. Only the first day draws them, as edge Labels of its inner Plot, so the numbers are part of the chart rather than a separate axis.
• The pointer’s choice goes back in as data. Each hour’s slat carries data-day and data-hour. The slat also covers the 1px gap around its Cell, so moving from one cell to the next never passes over a spot that belongs to no hour. One handler on the poster reads them with closest, and the picked day and hour are passed to the Plot, so every cell decides for itself whether to stay lit. The hour under the pointer is read out in the poster’s dek, on one line, so the chart never moves. See interaction for when to pass a choice back like this and when to draw a single element instead.