Adds the ability to draw an XY scatter plot. (#27)

* works more or less

* updating comment

* removing debugging printf

* adding output

* tweaks

* missed a couple series validations

* testing auto coloring

* updated output.png

* color tests etc.

* sanity check tests.

* should not use unkeyed fields anyway.

_examples/scatter/main.go

@@ -0,0 +1,80 @@
+package main
+
+import (
+	"log"
+	"net/http"
+
+	"github.com/wcharczuk/go-chart"
+)
+
+func drawChart(res http.ResponseWriter, req *http.Request) {
+	graph := chart.Chart{
+		Series: []chart.Series{
+			chart.ContinuousSeries{
+				Style: chart.Style{
+					Show:        true,
+					StrokeWidth: chart.Disabled,
+					DotWidth:    3,
+				},
+				XValues: chart.Sequence.Random(32, 1024),
+				YValues: chart.Sequence.Random(32, 1024),
+			},
+			chart.ContinuousSeries{
+				Style: chart.Style{
+					Show:        true,
+					StrokeWidth: chart.Disabled,
+					DotWidth:    5,
+				},
+				XValues: chart.Sequence.Random(16, 1024),
+				YValues: chart.Sequence.Random(16, 1024),
+			},
+			chart.ContinuousSeries{
+				Style: chart.Style{
+					Show:        true,
+					StrokeWidth: chart.Disabled,
+					DotWidth:    7,
+				},
+				XValues: chart.Sequence.Random(8, 1024),
+				YValues: chart.Sequence.Random(8, 1024),
+			},
+		},
+	}
+
+	res.Header().Set("Content-Type", "image/png")
+	err := graph.Render(chart.PNG, res)
+	if err != nil {
+		log.Println(err.Error())
+	}
+
+}
+
+func unit(res http.ResponseWriter, req *http.Request) {
+	graph := chart.Chart{
+		Height: 50,
+		Width:  50,
+		Canvas: chart.Style{
+			Padding: chart.Box{IsSet: true},
+		},
+		Background: chart.Style{
+			Padding: chart.Box{IsSet: true},
+		},
+		Series: []chart.Series{
+			chart.ContinuousSeries{
+				XValues: chart.Sequence.Float64(0, 4, 1),
+				YValues: chart.Sequence.Float64(0, 4, 1),
+			},
+		},
+	}
+
+	res.Header().Set("Content-Type", "image/png")
+	err := graph.Render(chart.PNG, res)
+	if err != nil {
+		log.Println(err.Error())
+	}
+}
+
+func main() {
+	http.HandleFunc("/", drawChart)
+	http.HandleFunc("/unit", unit)
+	log.Fatal(http.ListenAndServe(":8080", nil))
+}