adding donut type chart, like a pie chart with a blank circle on the center and little trick for label position (#111)

(some way of improvement)

_examples/donut_chart/main.go

@@ -0,0 +1,54 @@
+package main
+
+import (
+	"fmt"
+	"log"
+	"net/http"
+
+	"github.com/wcharczuk/go-chart"
+)
+
+func drawChart(res http.ResponseWriter, req *http.Request) {
+	pie := chart.DonutChart{
+		Width:  512,
+		Height: 512,
+		Values: []chart.Value{
+			{Value: 5, Label: "Blue"},
+			{Value: 5, Label: "Green"},
+			{Value: 4, Label: "Gray"},
+			{Value: 4, Label: "Orange"},
+			{Value: 3, Label: "Deep Blue"},
+			{Value: 3, Label: "test"},
+		},
+	}
+
+	res.Header().Set("Content-Type", "image/png")
+	err := pie.Render(chart.PNG, res)
+	if err != nil {
+		fmt.Printf("Error rendering pie chart: %v\n", err)
+	}
+}
+
+func drawChartRegression(res http.ResponseWriter, req *http.Request) {
+	pie := chart.DonutChart{
+		Width:  512,
+		Height: 512,
+		Values: []chart.Value{
+			{Value: 5, Label: "Blue"},
+			{Value: 2, Label: "Two"},
+			{Value: 1, Label: "One"},
+		},
+	}
+
+	res.Header().Set("Content-Type", chart.ContentTypeSVG)
+	err := pie.Render(chart.SVG, res)
+	if err != nil {
+		fmt.Printf("Error rendering pie chart: %v\n", err)
+	}
+}
+
+func main() {
+	http.HandleFunc("/", drawChart)
+	http.HandleFunc("/reg", drawChartRegression)
+	log.Fatal(http.ListenAndServe(":8080", nil))
+}