A program that draws github-like contribution graph chart

contributionChart.go

package main

import (
	"fmt"
	"image"
	"image/color"
	"image/draw"
	"image/png"
	"math/rand"
	"os"
	"time"
)

var margin = 3
var width, height = 1240, 480
var blockSize = 10
var withMarginSize = blockSize + margin
var img = image.NewRGBA(image.Rect(0, 0, width, height))
var light_gray = color.RGBA{0xee, 0xee, 0xee, 0xff}
var light_green = color.RGBA{0xc6, 0xe4, 0x8b, 0xff}  //c6e48b
var green = color.RGBA{0x7b, 0xc9, 0x6f, 0xff}        //7bc96f
var greener = color.RGBA{0x23, 0x9a, 0x3b, 0xff}      //239a3b
var the_greenest = color.RGBA{0x19, 0x61, 0x27, 0xff} //196127
var gitColors = []color.RGBA{light_green, green, greener, the_greenest}
var random = rand.New(rand.NewSource(time.Now().Unix()))

func getRandomColor() *color.RGBA {
	return &gitColors[random.Intn(len(gitColors))]
}
func main() {
	draw.Draw(img, img.Bounds(), &image.Uniform{light_gray}, image.ZP, draw.Src)
	for x := 0; x < width/withMarginSize; x++ {
		posx := x*blockSize + (x+1)*margin
		for y := 0; y < height/withMarginSize; y++ {
			fmt.Printf("Drawing rect %d", x+y)
			posy := y*blockSize + (y+1)*margin
			git_rect := image.Rect(posx, posy, posx+blockSize, posy+blockSize) //  geometry of 2nd rectangle
			draw.Draw(img, git_rect, &image.Uniform{getRandomColor()}, image.ZP, draw.Src)
		}
	}
	f, err := os.Create("draw.png")
	if err != nil {
		panic(err)
	}
	defer f.Close()
	png.Encode(f, img)
}