Added BaseValue funtionality to bar chart (#94)

_examples/bar_chart_base_value/main.go

@@ -0,0 +1,88 @@
+package main
+
+import (
+	"fmt"
+	"log"
+	"net/http"
+	"os"
+
+	"github.com/wcharczuk/go-chart"
+	"github.com/wcharczuk/go-chart/drawing"
+)
+
+func drawChart(res http.ResponseWriter, req *http.Request) {
+	profitStyle := chart.Style{
+		Show:        true,
+		FillColor:   drawing.ColorFromHex("13c158"),
+		StrokeColor: drawing.ColorFromHex("13c158"),
+		StrokeWidth: 0,
+	}
+
+	lossStyle := chart.Style{
+		Show:        true,
+		FillColor:   drawing.ColorFromHex("c11313"),
+		StrokeColor: drawing.ColorFromHex("c11313"),
+		StrokeWidth: 0,
+	}
+
+	sbc := chart.BarChart{
+		Title:      "Bar Chart Using BaseValue",
+		TitleStyle: chart.StyleShow(),
+		Background: chart.Style{
+			Padding: chart.Box{
+				Top: 40,
+			},
+		},
+		Height:   512,
+		BarWidth: 60,
+		XAxis: chart.Style{
+			Show: true,
+		},
+		YAxis: chart.YAxis{
+			Style: chart.Style{
+				Show: true,
+			},
+			Ticks: []chart.Tick{
+				{-4.0, "-4"},
+				{-2.0, "-2"},
+				{0, "0"},
+				{2.0, "2"},
+				{4.0, "4"},
+				{6.0, "6"},
+				{8.0, "8"},
+				{10.0, "10"},
+				{12.0, "12"},
+			},
+		},
+		UseBaseValue: true,
+		BaseValue:    0.0,
+		Bars: []chart.Value{
+			{Value: 10.0, Style: profitStyle, Label: "Profit"},
+			{Value: 12.0, Style: profitStyle, Label: "More Profit"},
+			{Value: 8.0, Style: profitStyle, Label: "Still Profit"},
+			{Value: -4.0, Style: lossStyle, Label: "Loss!"},
+			{Value: 3.0, Style: profitStyle, Label: "Phew Ok"},
+			{Value: -2.0, Style: lossStyle, Label: "Oh No!"},
+		},
+	}
+
+	res.Header().Set("Content-Type", "image/png")
+	err := sbc.Render(chart.PNG, res)
+	if err != nil {
+		fmt.Printf("Error rendering chart: %v\n", err)
+	}
+}
+
+func port() string {
+	if len(os.Getenv("PORT")) > 0 {
+		return os.Getenv("PORT")
+	}
+	return "8080"
+}
+
+func main() {
+	listenPort := fmt.Sprintf(":%s", port())
+	fmt.Printf("Listening on %s\n", listenPort)
+	http.HandleFunc("/", drawChart)
+	log.Fatal(http.ListenAndServe(listenPort, nil))
+}

bar_chart.go

@@ -31,6 +31,9 @@ type BarChart struct {
 
 	BarSpacing int
 
+	UseBaseValue bool
+	BaseValue    float64
+
 	Font        *truetype.Font
 	defaultFont *truetype.Font
 
@@ -199,11 +202,20 @@ func (bc BarChart) drawBars(r Renderer, canvasBox Box, yr Range) {
 
 		by = canvasBox.Bottom - yr.Translate(bar.Value)
 
-		barBox = Box{
-			Top:    by,
-			Left:   bxl,
-			Right:  bxr,
-			Bottom: canvasBox.Bottom,
+		if bc.UseBaseValue {
+			barBox = Box{
+				Top:    by,
+				Left:   bxl,
+				Right:  bxr,
+				Bottom: canvasBox.Bottom - yr.Translate(bc.BaseValue),
+			}
+		} else {
+			barBox = Box{
+				Top:    by,
+				Left:   bxl,
+				Right:  bxr,
+				Bottom: canvasBox.Bottom,
+			}
 		}
 
 		Draw.Box(r, barBox, bar.Style.InheritFrom(bc.styleDefaultsBar(index)))