first shot at gradient color grid.

package squares

import (

	"image/color"

	"math"

	"net/http"

	"github.com/ajstarks/svgo"

	"github.com/taironas/tinygraphs/draw"

)

// GradientSVG builds an image with 6 by 6 quadrants of alternate colors.

func GradientSVG(w http.ResponseWriter, key string, colors []color.RGBA, size int) {

	rainbow := []svg.Offcolor{

		{10, "#00cc00", 1},

		{30, "#006600", 1},

		{70, "#cc0000", 1},

		{90, "#000099", 1}}

	canvas := svg.New(w)

	canvas.Start(size, size)

	canvas.Def()

	canvas.LinearGradient("rainbow", 0, 0, uint8(size), 0, rainbow)

	canvas.DefEnd()

	canvas.Rect(0, 0, size, size, "fill:url(#rainbow)")

	squares := 6

	quadrantSize := size / squares

	middle := math.Ceil(float64(squares) / float64(2))

	colorMap := make(map[int]color.RGBA)

	colorIndex := make(map[int]int)

	for yQ := 0; yQ < squares; yQ++ {

		y := yQ * quadrantSize

		colorMap = make(map[int]color.RGBA)

		colorIndex = make(map[int]int)

		for xQ := 0; xQ < squares; xQ++ {

			x := xQ * quadrantSize

			fill := ""

			if _, ok := colorMap[xQ]; !ok {

				if float64(xQ) < middle {

					colorIndex[xQ] = draw.PickIndex(key, len(colors), xQ+3*yQ)

					colorMap[xQ] = draw.PickColor(key, colors, xQ+3*yQ)

				} else if xQ < squares {

					colorIndex[xQ] = colorIndex[squares-xQ-1]

					colorMap[xQ] = colorMap[squares-xQ-1]

				} else {

					colorIndex[xQ] = colorIndex[0]

					colorMap[xQ] = colorMap[0]

				}

			}

			if colorIndex[xQ] != 0 {

				fill = "fill:none; opacity:0.3"

			} else {

				fill = draw.FillFromRGBA(colorMap[xQ])

			}

			canvas.Rect(x, y, quadrantSize, quadrantSize, fill)

		}

	}

	canvas.End()

}