feat: implement legend type with fixed length line on the left side

fixes #185

legend.go

@@ -118,6 +118,124 @@ func Legend(c *Chart, userDefaults ...Style) Renderable {
 	}
 }
 
+// LegendLineLeft is a legend with the line drawn left to the legend text.
+func LegendLineLeft(c *Chart, userDefaults ...Style) Renderable {
+	return func(r Renderer, cb Box, chartDefaults Style) {
+		legendDefaults := Style{
+			FillColor:   drawing.ColorWhite,
+			FontColor:   DefaultTextColor,
+			FontSize:    8.0,
+			StrokeColor: DefaultAxisColor,
+			StrokeWidth: DefaultAxisLineWidth,
+		}
+
+		var legendStyle Style
+		if len(userDefaults) > 0 {
+			legendStyle = userDefaults[0].InheritFrom(chartDefaults.InheritFrom(legendDefaults))
+		} else {
+			legendStyle = chartDefaults.InheritFrom(legendDefaults)
+		}
+
+		// DEFAULTS
+		legendPadding := Box{
+			Top:    5,
+			Left:   5,
+			Right:  5,
+			Bottom: 5,
+		}
+		lineTextGap := 5
+		lineLengthMinimum := 25
+
+		var labels []string
+		var lines []Style
+		for index, s := range c.Series {
+			if !s.GetStyle().Hidden {
+				if _, isAnnotationSeries := s.(AnnotationSeries); !isAnnotationSeries {
+					labels = append(labels, s.GetName())
+					lines = append(lines, s.GetStyle().InheritFrom(c.styleDefaultsSeries(index)))
+				}
+			}
+		}
+
+		legend := Box{
+			Top:  cb.Top,
+			Left: cb.Left,
+			// bottom and right will be sized by the legend content + relevant padding.
+		}
+
+		legendContent := Box{
+			Top:    legend.Top + legendPadding.Top,
+			Left:   legend.Left + legendPadding.Left,
+			Right:  legend.Left + legendPadding.Left,
+			Bottom: legend.Top + legendPadding.Top,
+		}
+
+		legendStyle.GetTextOptions().WriteToRenderer(r)
+
+		// measure
+		labelCount := 0
+		for x := 0; x < len(labels); x++ {
+			if len(labels[x]) > 0 {
+				tb := r.MeasureText(labels[x])
+				if labelCount > 0 {
+					legendContent.Bottom += DefaultMinimumTickVerticalSpacing
+				}
+				legendContent.Bottom += tb.Height()
+				right := legendContent.Left + tb.Width() + lineTextGap + lineLengthMinimum
+				legendContent.Right = MaxInt(legendContent.Right, right)
+				labelCount++
+			}
+		}
+
+		legend = legend.Grow(legendContent)
+		legend.Right = legendContent.Right + legendPadding.Right
+		legend.Bottom = legendContent.Bottom + legendPadding.Bottom
+
+		Draw.Box(r, legend, legendStyle)
+
+		legendStyle.GetTextOptions().WriteToRenderer(r)
+
+		ycursor := legendContent.Top
+		lx := legendContent.Left
+		legendCount := 0
+		var label string
+		for x := 0; x < len(labels); x++ {
+			label = labels[x]
+			if len(label) > 0 {
+				if legendCount > 0 {
+					ycursor += DefaultMinimumTickVerticalSpacing
+				}
+
+				// Calculate text dimensions
+				tb := r.MeasureText(label)
+				ty := ycursor + tb.Height()
+				th2 := tb.Height() >> 1
+
+				// Calculate line x and y coordinates
+				ly := ty - th2
+
+				// Calculate line ending x coordinate
+				lx2 := lx + lineLengthMinimum
+
+				r.SetStrokeColor(lines[x].GetStrokeColor())
+				r.SetStrokeWidth(lines[x].GetStrokeWidth())
+				r.SetStrokeDashArray(lines[x].GetStrokeDashArray())
+
+				r.MoveTo(lx, ly)
+				r.LineTo(lx2, ly)
+				r.Stroke()
+
+				// Calculate Text starting coordinates
+				textX := lx2 + lineTextGap
+				r.Text(label, textX, ty)
+
+				ycursor += tb.Height()
+				legendCount++
+			}
+		}
+	}
+}
+
 // LegendThin is a legend that doesn't obscure the chart area.
 func LegendThin(c *Chart, userDefaults ...Style) Renderable {
 	return func(r Renderer, cb Box, chartDefaults Style) {