What TanStack Charts does for heatmaps on its own, and what you add so that everyone can read them.
What you get
A named chart. ariaLabel is required, and the chart is exposed as an image with that name. ariaDescription adds a longer description.
Keyboard navigation, on by default. Tab moves into the chart and focuses the first cell. Arrow keys move between them, and Home and End jump to the first and the last. Enter or Space pins a sticky tooltip, and Escape closes it.
The tooltip is announced to screen readers as it changes, so a keyboard user hears the values of the focused cell.
A visible focus ring in the theme's focus color, shown for keyboard focus only.
Reduced motion. Animated charts skip their animation when the reader has asked for less motion.
An interactive legend made of real buttons, each with a pressed state, when you use interactiveColorLegend.
What you add
A label that says what the chart shows, its unit and its period. "Site visits by weekday and hour, last 4 weeks" works. "Heatmap" does not.
A visible title, and a subtitle with the unit and the period, in HTML next to the chart.
The exact values another way, when they matter: a table under the chart, behind a toggle, or a download.
A second signal besides color for every series or part.
A keyboard path for every control you add yourself, such as a series filter or a period picker.
Name and describe the chart
Keep the label short and put the takeaway in the description. Do not put every value into the name: a screen reader reads it in one breath.
<Chart
definition={definition}
ariaLabel="Site visits by weekday and hour, last 4 weeks"
ariaDescription="Visits peak on weekday mornings from 9 to 11 and drop to almost none after 8 in the evening."
/>
Give the values as a table
A table is the most reliable way to give exact values to everyone, including people who zoom far in or print the page. Render the same array the chart reads with the kit's Table, under the chart or behind a "Show as table" switch. When the values are only an illustration, a one sentence summary is enough.
Do not rely on color
Use a scale that changes in lightness, from light to dark, so the order survives in grayscale and for color blind readers. Avoid red against green and rainbow scales.
Print the value in each cell when the cells are large enough, so no one has to match a shade to the legend.
Give missing cells a neutral fill clearly apart from the lowest step, or a dash as their label, and say what it means.
Marks need enough contrast with the background. Check light colors from a palette against the page in light and dark mode.
A legend everyone can read
The legend TanStack draws from colorLegend or colorGradientLegend is hidden from screen readers. Write the steps and the unit next to the chart in HTML as well, such as "Lighter is fewer visits, darker is more, in steps of 50."
Many cells
A year of daily cells is 365 stops for the arrow keys, and each one is a single number out of context. Give the pattern first.
Put the takeaway in ariaDescription: where the values are highest, where they are lowest, and any gaps.
Offer the values as a table with the same rows and columns, or totals per row and column.
Turning the keyboard off
A static chart with no tooltip can pass keyboard: false in the second argument of defineChart, so it does not add a tab stop. Keep it on for any chart that shows values on hover: otherwise keyboard users never see them.
Check it
Tab into the chart and read every cell with the arrow keys.
Listen to it with a screen reader.
Zoom the page to 400% and confirm the values are still there.
View it in grayscale and confirm the series are still apart.
The Accessibility guideline has the principles that apply to every chart.