From 8310896cd0cd0233cb26c521caa4156290ed7ad4 Mon Sep 17 00:00:00 2001
From: Alan Bernstein <alan.aaron.bernstein@gmail.com>
Date: Thu, 20 Aug 2020 23:29:13 -0500
Subject: [PATCH] Add ability to draw bar chart X-axis at BaseValue

---
 bar_chart.go                          | 27 ++++++++++++++++-----------
 examples/bar_chart_base_value/main.go |  6 ++++--
 2 files changed, 20 insertions(+), 13 deletions(-)

diff --git a/bar_chart.go b/bar_chart.go
index d61f3db..f41e1eb 100644
--- a/bar_chart.go
+++ b/bar_chart.go
@@ -30,8 +30,9 @@ type BarChart struct {
 
 	BarSpacing int
 
-	UseBaseValue bool
-	BaseValue    float64
+	UseBaseValue      bool
+	BaseValue         float64
+	UseBaseValueXAxis bool
 
 	Font        *truetype.Font
 	defaultFont *truetype.Font
@@ -130,7 +131,7 @@ func (bc BarChart) Render(rp RendererProvider, w io.Writer) error {
 	}
 	bc.drawCanvas(r, canvasBox)
 	bc.drawBars(r, canvasBox, yr)
-	bc.drawXAxis(r, canvasBox)
+	bc.drawXAxis(r, canvasBox, yr)
 	bc.drawYAxis(r, canvasBox, yr, yt)
 
 	bc.drawTitle(r)
@@ -223,25 +224,29 @@ func (bc BarChart) drawBars(r Renderer, canvasBox Box, yr Range) {
 	}
 }
 
-func (bc BarChart) drawXAxis(r Renderer, canvasBox Box) {
+func (bc BarChart) drawXAxis(r Renderer, canvasBox Box, yr Range) {
 	if !bc.XAxis.Hidden {
 		axisStyle := bc.XAxis.InheritFrom(bc.styleDefaultsAxes())
 		axisStyle.WriteToRenderer(r)
 
 		width, spacing, _ := bc.calculateScaledTotalWidth(canvasBox)
 
-		r.MoveTo(canvasBox.Left, canvasBox.Bottom)
-		r.LineTo(canvasBox.Right, canvasBox.Bottom)
+		axisYValue := canvasBox.Bottom
+		if bc.UseBaseValueXAxis {
+			axisYValue -= yr.Translate(bc.BaseValue)
+		}
+		r.MoveTo(canvasBox.Left, axisYValue)
+		r.LineTo(canvasBox.Right, axisYValue)
 		r.Stroke()
 
-		r.MoveTo(canvasBox.Left, canvasBox.Bottom)
-		r.LineTo(canvasBox.Left, canvasBox.Bottom+DefaultVerticalTickHeight)
+		r.MoveTo(canvasBox.Left, axisYValue)
+		r.LineTo(canvasBox.Left, axisYValue+DefaultVerticalTickHeight)
 		r.Stroke()
 
 		cursor := canvasBox.Left
 		for index, bar := range bc.Bars {
 			barLabelBox := Box{
-				Top:    canvasBox.Bottom + DefaultXAxisMargin,
+				Top:    axisYValue + DefaultXAxisMargin,
 				Left:   cursor,
 				Right:  cursor + width + spacing,
 				Bottom: bc.GetHeight(),
@@ -253,8 +258,8 @@ func (bc BarChart) drawXAxis(r Renderer, canvasBox Box) {
 
 			axisStyle.WriteToRenderer(r)
 			if index < len(bc.Bars)-1 {
-				r.MoveTo(barLabelBox.Right, canvasBox.Bottom)
-				r.LineTo(barLabelBox.Right, canvasBox.Bottom+DefaultVerticalTickHeight)
+				r.MoveTo(barLabelBox.Right, axisYValue)
+				r.LineTo(barLabelBox.Right, axisYValue+DefaultVerticalTickHeight)
 				r.Stroke()
 			}
 			cursor += width + spacing
diff --git a/examples/bar_chart_base_value/main.go b/examples/bar_chart_base_value/main.go
index 84ad9c4..14adbe8 100644
--- a/examples/bar_chart_base_value/main.go
+++ b/examples/bar_chart_base_value/main.go
@@ -31,6 +31,7 @@ func main() {
 		},
 		Height:   512,
 		BarWidth: 60,
+		XAxis:    chart.Style{StrokeColor: drawing.ColorBlack, StrokeWidth: 1.0},
 		YAxis: chart.YAxis{
 			Ticks: []chart.Tick{
 				{Value: -4.0, Label: "-4"},
@@ -44,8 +45,9 @@ func main() {
 				{Value: 12.0, Label: "12"},
 			},
 		},
-		UseBaseValue: true,
-		BaseValue:    0.0,
+		UseBaseValue:   true,
+		BaseValueXAxis: true,
+		BaseValue:      0.0,
 		Bars: []chart.Value{
 			{Value: 10.0, Style: profitStyle, Label: "Profit"},
 			{Value: 12.0, Style: profitStyle, Label: "More Profit"},