Adds the ability to draw an XY scatter plot. (#27)

* works more or less

* updating comment

* removing debugging printf

* adding output

* tweaks

* missed a couple series validations

* testing auto coloring

* updated output.png

* color tests etc.

* sanity check tests.

* should not use unkeyed fields anyway.

drawing/color_test.go

@@ -3,6 +3,8 @@ package drawing
 import (
 	"testing"
 
+	"image/color"
+
 	"github.com/blendlabs/go-assert"
 )
 
@@ -39,3 +41,13 @@ func TestColorFromHex(t *testing.T) {
 	shortBlue := ColorFromHex("00F")
 	assert.Equal(ColorBlue, shortBlue)
 }
+
+func TestColorFromAlphaMixedRGBA(t *testing.T) {
+	assert := assert.New(t)
+
+	black := ColorFromAlphaMixedRGBA(color.Black.RGBA())
+	assert.True(black.Equals(ColorBlack), black.String())
+
+	white := ColorFromAlphaMixedRGBA(color.White.RGBA())
+	assert.True(white.Equals(ColorWhite), white.String())
+}