feat: support detect color dark or light

mark_point.go

@@ -24,7 +24,6 @@ package charts
 
 import (
 	"github.com/golang/freetype/truetype"
-	"github.com/wcharczuk/go-chart/v2/drawing"
 )
 
 // NewMarkPoint returns a series mark point
@@ -78,16 +77,15 @@ func (m *markPointPainter) Render() (Box, error) {
 			symbolSize = 30
 		}
 		textStyle := Style{
-			FontColor: drawing.Color{
-				R: 238,
-				G: 238,
-				B: 238,
-				A: 255,
-			},
 			FontSize:    labelFontSize,
 			StrokeWidth: 1,
 			Font:        opt.Font,
 		}
+		if isLightColor(opt.FillColor) {
+			textStyle.FontColor = defaultLightFontColor
+		} else {
+			textStyle.FontColor = defaultDarkFontColor
+		}
 		painter.OverrideDrawingStyle(Style{
 			FillColor: opt.FillColor,
 		}).OverrideTextStyle(textStyle)

theme.go

@@ -76,6 +76,19 @@ const defaultFontSize = 12.0
 
 var defaultTheme ColorPalette
 
+var defaultLightFontColor = drawing.Color{
+	R: 70,
+	G: 70,
+	B: 70,
+	A: 255,
+}
+var defaultDarkFontColor = drawing.Color{
+	R: 238,
+	G: 238,
+	B: 238,
+	A: 255,
+}
+
 func init() {
 	echartSeriesColors := []Color{
 		parseColor("#5470c6"),

util.go

@@ -262,3 +262,10 @@ func getPolygonPoints(center Point, radius float64, sides int) []Point {
 	}
 	return points
 }
+
+func isLightColor(c Color) bool {
+	r := float64(c.R) * float64(c.R) * 0.299
+	g := float64(c.G) * float64(c.G) * 0.587
+	b := float64(c.B) * float64(c.B) * 0.114
+	return math.Sqrt(r+g+b) > 127.5
+}

util_test.go

@@ -189,3 +189,35 @@ func TestParseColor(t *testing.T) {
 		A: 250,
 	}, c)
 }
+
+func TestIsLightColor(t *testing.T) {
+	assert := assert.New(t)
+
+	assert.True(isLightColor(drawing.Color{
+		R: 255,
+		G: 255,
+		B: 255,
+	}))
+	assert.True(isLightColor(drawing.Color{
+		R: 145,
+		G: 204,
+		B: 117,
+	}))
+
+	assert.False(isLightColor(drawing.Color{
+		R: 88,
+		G: 112,
+		B: 198,
+	}))
+
+	assert.False(isLightColor(drawing.Color{
+		R: 0,
+		G: 0,
+		B: 0,
+	}))
+	assert.False(isLightColor(drawing.Color{
+		R: 16,
+		G: 12,
+		B: 42,
+	}))
+}