refactor: enhance legend render function

chart.go

@@ -81,6 +81,7 @@ func (o *ChartOption) FillDefault(t *Theme) {
 			Bottom: 5,
 		}
 	}
+	o.Legend.Theme = t
 	if o.Legend.Style.FontSize == 0 {
 		o.Legend.Style.FontSize = 8
 	}

legend.go

@@ -27,20 +27,35 @@ import (
 )
 
 type LegendOption struct {
+	Theme *Theme
 	Style chart.Style
 	Data  []string
 	Left  string
 	Right string
 	Align string
 }
+type legend struct {
+	d   *Draw
+	opt *LegendOption
+}
 
-func drawLegend(p *Draw, opt *LegendOption, theme *Theme) (chart.Box, error) {
+func NewLegend(d *Draw, opt LegendOption) *legend {
+	return &legend{
+		d:   d,
+		opt: &opt,
+	}
+}
+
+func (l *legend) Render() (chart.Box, error) {
+	d := l.d
+	opt := l.opt
 	if len(opt.Data) == 0 {
 		return chart.BoxZero, nil
 	}
+	theme := opt.Theme
 	padding := opt.Style.Padding
 	legendDraw, err := NewDraw(DrawOption{
-		Parent: p,
+		Parent: d,
 	}, PaddingOption(padding))
 	if err != nil {
 		return chart.BoxZero, err

line_chart.go

@@ -58,7 +58,7 @@ func NewLineChart(opt LineChartOption) (*Draw, error) {
 		return nil, err
 	}
 
-	_, err = drawLegend(d, &opt.Legend, &theme)
+	_, err = NewLegend(d, opt.Legend).Render()
 	if err != nil {
 		return nil, err
 	}