Calendar Heatmap Chart
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Free Online Calendar Heatmap Chart Maker
What is a Calendar Heatmap Chart?
A calendar heatmap chart maps one numeric value per day onto a calendar grid, using color intensity to reveal activity patterns across weeks, months, or a full year. It is the format made popular by GitHub's contribution graph — adapted for any daily time-series data. Use it when you want readers to spot busy periods, seasonal rhythms, and day-of-week patterns in a single glance rather than scanning a long line chart.
Key Features
Color-coded intensity
Each day cell is shaded from light to dark based on its value, so high- and low-activity periods stand out without labels.
Full calendar grid layout
Weeks run column by column with month labels above and day-of-week labels on the side, preserving the familiar calendar structure.
Five color palettes
Switch between green, blue, orange, purple, and red palettes to match your brand or the nature of your data.
Adjustable cell size and spacing
Control cell size, corner radius, and gap between cells to balance density and readability for slides, dashboards, or reports.
Hover tooltips
Each cell shows the exact date and value on hover, letting readers inspect individual days without cluttering the grid.
Toggle labels and legend
Show or hide month labels, day labels, and the color legend to simplify the chart for any audience or format.
Best For
When to Use
- You have one numeric value per day across weeks, months, or a full year
- You want to expose weekly or seasonal patterns that a line chart would compress into noise
- Your audience needs to identify specific dates with unusually high or low values
- Day-of-week patterns — such as weekend dips vs. weekday peaks — are central to your story
- You are comparing activity density across multiple years side by side
Common Mistakes
- !Using only two color steps — a binary palette loses nuance; use at least four intensity levels
- !Plotting multiple metrics on one grid — one value per day keeps the color encoding unambiguous
- !Leaving zero-value days blank — empty cells imply missing data, not zero; use a pale fill for zero
- !Starting the grid mid-week — always begin on a consistent day (Monday or Sunday) to preserve the visual rhythm
- !Omitting the color legend — readers cannot interpret intensity without knowing what 'more' and 'less' represent
- !Using this chart for sparse or irregular data — if most days are empty, a bar or scatter chart communicates the gaps more honestly