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

Basic
![basic](charts/testdata/basic.png)

Colorscales
![col1](charts/testdata/colorscale_1.png)
![col2](charts/testdata/colorscale_2.png)

Without month separator
![nosep](charts/testdata/noseparator.png)

Without labels
![nolab](charts/testdata/nolabels.png)

Without labels, without separator
![nosep_nolab](charts/testdata/noseparator_nolabels.png)

Example module, next save output in formats supported by `image` module (PNG, JPEG, GIF).

```go
countByDayOfYear := map[int]int{
    1: 10,
    22: 15,
    150: 22,
    366: 55,
    ...
}

img := charts.NewHeatmap(charts.HeatmapConfig{
    Year:               2020,
    CountByDay:         countByDay,
    ColorScale:         colorscales.PuBu9,
    DrawMonthSeparator: true,
    DrawLabels:         true,
    ...
})

```

Example script, will output as PNG.
```
$ go build; ./calendarheatmap -h

Usage of ./calendarheatmap:
  -colorscale string
        refer to colorscales for examples (default "PuBu9")
  -input string
        file should contain lines in format: 2020-05-16 20:43 PPPP (default "input.txt")
  -labels
        labels for weekday and months (default true)
  -monthsep
        render month separator (default true)
  -output string
        output filename, will export as PNG (default "chart.png")
  -output-format string
        output format (png, jpeg, gif) (default "png"))
```

TODO:
- [ ] SVG support
- [ ] select start and end date
- [ ] CSV, JSON in script input