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.go

@@ -46,12 +46,20 @@ func ColorFromHex(hex string) Color {
 	return c
 }
 
+// ColorFromAlphaMixedRGBA returns the system alpha mixed rgba values.
+func ColorFromAlphaMixedRGBA(r, g, b, a uint32) Color {
+	fa := float64(a) / 255.0
+	var c Color
+	c.R = uint8(float64(r) / fa)
+	c.G = uint8(float64(g) / fa)
+	c.B = uint8(float64(b) / fa)
+	c.A = uint8(a | (a >> 8))
+	return c
+}
+
 // Color is our internal color type because color.Color is bullshit.
 type Color struct {
-	R uint8
-	G uint8
-	B uint8
-	A uint8
+	R, G, B, A uint8
 }
 
 // RGBA returns the color as a pre-alpha mixed color set.
@@ -88,6 +96,24 @@ func (c Color) WithAlpha(a uint8) Color {
 	}
 }
 
+// Equals returns true if the color equals another.
+func (c Color) Equals(other Color) bool {
+	return c.R == other.R &&
+		c.G == other.G &&
+		c.B == other.B &&
+		c.A == other.A
+}
+
+// AverageWith averages two colors.
+func (c Color) AverageWith(other Color) Color {
+	return Color{
+		R: (c.R + other.R) >> 1,
+		G: (c.G + other.G) >> 1,
+		B: (c.B + other.B) >> 1,
+		A: c.A,
+	}
+}
+
 // String returns a css string representation of the color.
 func (c Color) String() string {
 	fa := float64(c.A) / float64(255)