Stop 05 · Data VisualizationTopics · 8/10

Heatmaps & Treemaps

In short

When a table has 168 numbers (7 days × 24 hours), nobody reads it. A heatmap turns it into a picture where busy times glow. Treemaps do something similar for nested categories.

A heatmap uses color intensity to show magnitude across two categorical dimensions (e.g. day of week vs hour); a treemap shows hierarchical part-to-whole relationships using nested rectangles.

The jaadu points at garden patches shaded light to dark violet, next to a divided vegetable crate.

Think of it like this

Patches in a kitchen garden

In a garden, darker patches show where plants grow thickest, so you see the lush corner without counting leaves. That's a heatmap. A vegetable crate divided into compartments sized by how much each holds is a treemap: big box, big share.

Watch it happen

Orders by hour on a Saturday

8am: 12

11am: 30

1pm: 85

4pm: 25

8pm: 120

11pm: 40

1/4One row of a day-by-hour heatmap for a food app. Numbers alone are slow to scan.

The big ideas

What you need to know

Heatmap: two categories, one value

Rows and columns are categories, like day and hour. Colour shows the value in each cell.

Use a sequential scale

Light to dark of one hue for counts. Diverging only if there's a meaningful middle.

Treemap: nested parts of a whole

Rectangles sized by value, nested by hierarchy, like category then sub-category.

Treemaps hide small things

Tiny rectangles become unreadable. Great for a few big shares, poor for precise comparison.

Save yourself the pain

Common mistakes

Oops

Using a rainbow scale on a heatmap.

Do this instead

Use one hue from light to dark so 'darker means more' is obvious.

Oops

Using a treemap when exact comparison matters.

Do this instead

Use a sorted bar chart. Area is much harder to compare than length.

Your mission

Map your week

~25 min

Quick check

You want to see which day-and-hour combinations get the most website traffic. Which chart?

Go deeper

1Build a heatmap showing activity by day-of-week and hour-of-day.
2Build a treemap showing revenue breakdown by category and sub-category.
3Learn when these specialized charts add real clarity versus just novelty.