Self-contained, plain Go implementation of calendar heatmap inspired by Github contribution activity.

$ go build
$ echo '{
    "2020-05-16": 8,
    "2020-05-17": 13,
    "2020-05-18": 5,
    "2020-05-19": 8,
    "2020-05-20": 5
}' | ./calendarheatmap > chart.png

Basic

Colorscales

UTF-8

SVG

Without month separator

Without labels

Without labels, without separator