feat: support legend render function

legend.go

@@ -0,0 +1,95 @@
+// MIT License
+
+// Copyright (c) 2022 Tree Xie
+
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+
+// The above copyright notice and this permission notice shall be included in all
+// copies or substantial portions of the Software.
+
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+// SOFTWARE.
+
+package charts
+
+import (
+	"github.com/wcharczuk/go-chart/v2"
+)
+
+type LegendOption struct {
+	Style chart.Style
+	Data  []string
+	Left  string
+	Right string
+	Align string
+}
+
+func drawLegend(p *Draw, opt *LegendOption, theme *Theme) (chart.Box, error) {
+	if len(opt.Data) == 0 {
+		return chart.BoxZero, nil
+	}
+	padding := opt.Style.Padding
+	legendDraw, err := NewDraw(DrawOption{
+		Parent: p,
+	}, PaddingOption(padding))
+	if err != nil {
+		return chart.BoxZero, err
+	}
+	r := legendDraw.Render
+	opt.Style.GetTextOptions().WriteToRenderer(r)
+
+	x := 0
+	y := 0
+	legendWidth := 30
+	legendDotHeight := 5
+	textPadding := 5
+	legendMargin := 10
+	for index, text := range opt.Data {
+		if index != 0 {
+			x += legendMargin
+		}
+		style := chart.Style{
+			StrokeColor: theme.GetSeriesColor(index),
+			FillColor:   theme.GetSeriesColor(index),
+			StrokeWidth: 2,
+		}
+		textBox := r.MeasureText(text)
+		renderText := func() {
+			x += textPadding
+			legendDraw.text(text, x, y+legendDotHeight-1)
+			x += textBox.Width()
+			x += textPadding
+		}
+
+		if opt.Align == PositionRight {
+			renderText()
+		}
+
+		style.GetFillAndStrokeOptions().WriteDrawingOptionsToRenderer(r)
+		legendDraw.moveTo(x, y)
+		legendDraw.lineTo(x+legendWidth, y)
+		r.Stroke()
+		legendDraw.circle(float64(legendDotHeight), x+legendWidth>>1, y)
+		r.FillStroke()
+		x += legendWidth
+
+		if opt.Align != PositionRight {
+			renderText()
+		}
+	}
+	legendBox := padding.Clone()
+	legendBox.Right = legendBox.Left + x
+	legendBox.Bottom = legendBox.Top + 2*legendDotHeight
+
+	return legendBox, nil
+}