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))
+}

_examples/donut_chart/reg.svg

@@ -0,0 +1,25 @@
+<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" width="512" height="512">\n<path  d="M 0 0
+L 512 0
+L 512 512
+L 0 512
+L 0 0" style="stroke-width:0;stroke:rgba(255,255,255,1.0);fill:rgba(255,255,255,1.0)"/><path  d="M 5 5
+L 507 5
+L 507 507
+L 5 507
+L 5 5" style="stroke-width:0;stroke:rgba(255,255,255,1.0);fill:rgba(255,255,255,1.0)"/><path  d="M 256 256
+L 438 256
+A 182 182 225.00 1 1 127 127
+L 256 256
+Z" style="stroke-width:4;stroke:rgba(255,255,255,1.0);fill:rgba(106,195,203,1.0)"/><path  d="M 256 256
+L 127 127
+A 182 182 90.00 0 1 385 127
+L 256 256
+Z" style="stroke-width:4;stroke:rgba(255,255,255,1.0);fill:rgba(42,190,137,1.0)"/><path  d="M 256 256
+L 385 127
+A 182 182 45.00 0 1 438 256
+L 256 256
+Z" style="stroke-width:4;stroke:rgba(255,255,255,1.0);fill:rgba(110,128,139,1.0)"/><path  d="M 256 256
+L 321 256
+A 65 65 359.00 1 1 321 255
+L 256 256
+Z" style="stroke-width:4;stroke:rgba(255,255,255,1.0);fill:rgba(255,255,255,1.0)"/><text x="159" y="461" style="stroke-width:0;stroke:none;fill:rgba(51,51,51,1.0);font-size:15.3px;font-family:'Roboto Medium',sans-serif">Blue</text><text x="241" y="48" style="stroke-width:0;stroke:none;fill:rgba(51,51,51,1.0);font-size:15.3px;font-family:'Roboto Medium',sans-serif">Two</text><text x="440" y="181" style="stroke-width:0;stroke:none;fill:rgba(51,51,51,1.0);font-size:15.3px;font-family:'Roboto Medium',sans-serif">One</text></svg>