Support URL link from the legend box. (#143)

Fixes #142.

internal/driver/testdata/pprof.cpu.flat.functions.call_tree.dot

 digraph "testbinary" {
 node [style=filled fillcolor="#f8f8f8"]
-subgraph cluster_L { "File: testbinary" [shape=box fontsize=16 label="File: testbinary\lType: cpu\lDuration: 10s, Total samples = 1.12s (11.20%)\lShowing nodes accounting for 1.11s, 99.11% of 1.12s total\lDropped 3 nodes (cum <= 0.06s)\l"] }
+subgraph cluster_L { "File: testbinary" [shape=box fontsize=16 label="File: testbinary\lType: cpu\lDuration: 10s, Total samples = 1.12s (11.20%)\lShowing nodes accounting for 1.11s, 99.11% of 1.12s total\lDropped 3 nodes (cum <= 0.06s)\l" tooltip="testbinary"] }
 N1 [label="line1000\n1s (89.29%)" fontsize=24 shape=box tooltip="line1000 (1s)" color="#b20500" fillcolor="#edd6d5"]
 N1_0 [label = "key1:tag1\nkey2:tag1" fontsize=8 shape=box3d tooltip="1s"]
 N1 -> N1_0 [label=" 1s" weight=100 tooltip="1s" labeltooltip="1s"]

internal/driver/testdata/pprof.cpu.flat.functions.dot

 digraph "testbinary" {
 node [style=filled fillcolor="#f8f8f8"]
-subgraph cluster_L { "File: testbinary" [shape=box fontsize=16 label="File: testbinary\lType: cpu\lDuration: 10s, Total samples = 1.12s (11.20%)\lShowing nodes accounting for 1.12s, 100% of 1.12s total\l"] }
+subgraph cluster_L { "File: testbinary" [shape=box fontsize=16 label="File: testbinary\lType: cpu\lDuration: 10s, Total samples = 1.12s (11.20%)\lShowing nodes accounting for 1.12s, 100% of 1.12s total\l" tooltip="testbinary"] }
 N1 [label="line1000\n1.10s (98.21%)" fontsize=24 shape=box tooltip="line1000 (1.10s)" color="#b20000" fillcolor="#edd5d5"]
 N1_0 [label = "key1:tag1\nkey2:tag1" fontsize=8 shape=box3d tooltip="1s"]
 N1 -> N1_0 [label=" 1s" weight=100 tooltip="1s" labeltooltip="1s"]

internal/graph/dotgraph.go

 // DotConfig contains attributes about how a graph should be
 // constructed and how it should look.
 type DotConfig struct {
-	Title  string   // The title of the DOT graph
-	Labels []string // The labels for the DOT's legend
+	Title     string   // The title of the DOT graph
+	LegendURL string   // The URL to link to from the legend.
+	Labels    []string // The labels for the DOT's legend
 
 	FormatValue func(int64) string         // A formatting function for values
 	FormatTag   func(int64, string) string // A formatting function for numeric tags
 	if len(labels) > 0 {
 		title = labels[0]
 	}
-	fmt.Fprintf(b, `subgraph cluster_L { "%s" [shape=box fontsize=16 label="%s\l"] }`+"\n", title, strings.Join(labels, `\l`))
+	fmt.Fprintf(b, `subgraph cluster_L { "%s" [shape=box fontsize=16`, title)
+	fmt.Fprintf(b, ` label="%s\l"`, strings.Join(labels, `\l`))
+	if b.config.LegendURL != "" {
+		fmt.Fprintf(b, ` URL="%s" target="_blank"`, b.config.LegendURL)
+	}
+	if b.config.Title != "" {
+		fmt.Fprintf(b, ` tooltip="%s"`, b.config.Title)
+	}
+	fmt.Fprintf(b, "] }\n")
 }
 
 // addNode generates a graph node in DOT format.

internal/graph/dotgraph_test.go

 	compareGraphs(t, buf.Bytes(), want)
 }
 
+func TestComposeWithStandardGraphAndURL(t *testing.T) {
+	g := baseGraph()
+	a, c := baseAttrsAndConfig()
+	c.LegendURL = "http://example.com"
+
+	var buf bytes.Buffer
+	ComposeDot(&buf, g, a, c)
+
+	want, err := ioutil.ReadFile(path + "compose6.dot")
+	if err != nil {
+		t.Fatalf("error reading test file: %v", err)
+	}
+
+	compareGraphs(t, buf.Bytes(), want)
+}
+
 func baseGraph() *Graph {
 	src := &Node{
 		Info:        NodeInfo{Name: "src"},

internal/graph/testdata/compose1.dot

 digraph "testtitle" {
 node [style=filled fillcolor="#f8f8f8"]
-subgraph cluster_L { "label1" [shape=box fontsize=16 label="label1\llabel2\l"] }
+subgraph cluster_L { "label1" [shape=box fontsize=16 label="label1\llabel2\l" tooltip="testtitle"] }
 N1 [label="src\n10 (10.00%)\nof 25 (25.00%)" fontsize=22 shape=box tooltip="src (25)" color="#b23c00" fillcolor="#edddd5"]
 N2 [label="dest\n15 (15.00%)\nof 25 (25.00%)" fontsize=24 shape=box tooltip="dest (25)" color="#b23c00" fillcolor="#edddd5"]
 N1 -> N2 [label=" 10" weight=11 color="#b28559" tooltip="src -> dest (10)" labeltooltip="src -> dest (10)"]

internal/graph/testdata/compose2.dot

 digraph "testtitle" {
 node [style=filled fillcolor="#f8f8f8"]
-subgraph cluster_L { "label1" [shape=box fontsize=16 label="label1\llabel2\l"] }
+subgraph cluster_L { "label1" [shape=box fontsize=16 label="label1\llabel2\l" tooltip="testtitle"] }
 N1 [label="SRC10 (10.00%)\nof 25 (25.00%)" fontsize=24 shape=folder tooltip="src (25)" color="#b23c00" fillcolor="#edddd5" style="bold,filled" peripheries=2 URL="www.google.com" target="_blank"]
 N2 [label="dest\n0 of 25 (25.00%)" fontsize=8 shape=box tooltip="dest (25)" color="#b23c00" fillcolor="#edddd5"]
 N1 -> N2 [label=" 10" weight=11 color="#b28559" tooltip="src -> dest (10)" labeltooltip="src -> dest (10)"]

internal/graph/testdata/compose3.dot

 digraph "testtitle" {
 node [style=filled fillcolor="#f8f8f8"]
-subgraph cluster_L { "label1" [shape=box fontsize=16 label="label1\llabel2\l"] }
+subgraph cluster_L { "label1" [shape=box fontsize=16 label="label1\llabel2\l" tooltip="testtitle"] }
 N1 [label="src\n10 (10.00%)\nof 25 (25.00%)" fontsize=22 shape=box tooltip="src (25)" color="#b23c00" fillcolor="#edddd5"]
 N1_0 [label = "tag1" fontsize=8 shape=box3d tooltip="10"]
 N1 -> N1_0 [label=" 10" weight=100 tooltip="10" labeltooltip="10"]

internal/graph/testdata/compose4.dot

 digraph "testtitle" {
 node [style=filled fillcolor="#f8f8f8"]
-subgraph cluster_L { "label1" [shape=box fontsize=16 label="label1\llabel2\l"] }
+subgraph cluster_L { "label1" [shape=box fontsize=16 label="label1\llabel2\l" tooltip="testtitle"] }
 }

internal/graph/testdata/compose5.dot

 digraph "testtitle" {
 node [style=filled fillcolor="#f8f8f8"]
-subgraph cluster_L { "label1" [shape=box fontsize=16 label="label1\llabel2\l"] }
+subgraph cluster_L { "label1" [shape=box fontsize=16 label="label1\llabel2\l" tooltip="testtitle"] }
 N1 [label="src\n10 (10.00%)\nof 25 (25.00%)" fontsize=22 shape=box tooltip="src (25)" color="#b23c00" fillcolor="#edddd5"]
 N1_0 [label = "tag1" fontsize=8 shape=box3d tooltip="10"]
 N1 -> N1_0 [label=" 10" weight=100 tooltip="10" labeltooltip="10"]

internal/graph/testdata/compose6.dot

+digraph "testtitle" {
+node [style=filled fillcolor="#f8f8f8"]
+subgraph cluster_L { "label1" [shape=box fontsize=16 label="label1\llabel2\l" URL="http://example.com" target="_blank" tooltip="testtitle"] }
+N1 [label="src\n10 (10.00%)\nof 25 (25.00%)" fontsize=22 shape=box tooltip="src (25)" color="#b23c00" fillcolor="#edddd5"]
+N2 [label="dest\n15 (15.00%)\nof 25 (25.00%)" fontsize=24 shape=box tooltip="dest (25)" color="#b23c00" fillcolor="#edddd5"]
+N1 -> N2 [label=" 10" weight=11 color="#b28559" tooltip="src -> dest (10)" labeltooltip="src -> dest (10)"]
+}