dashboard: further improve graph view

1. Don't interpolate missing data points,
instead just use the previous value for missing points.
Interpolation hides significant line jumps,
making it look like the jump is a result of several small changes.
2. Add annotations for significant changes.

LGTM=adg
R=adg
CC=golang-codereviews, iant, rsc
https://golang.org/cl/154360046

dashboard/app/build/perf_graph.go

@@ -133,6 +133,7 @@ func perfGraphHandler(w http.ResponseWriter, r *http.Request) {
 
 	var vals [][]float64
 	var hints [][]string
+	var annotations [][]string
 	var certainty [][]bool
 	var headers []string
 	commits2, err := GetCommits(c, startCommitNum, commitsToDisplay)
@@ -157,6 +158,7 @@ func perfGraphHandler(w http.ResponseWriter, r *http.Request) {
 						}
 					}
 					if hasdata {
+						noise := pc.NoiseLevel(builder, benchProcs, metric)
 						descBuilder := "/" + builder
 						descBenchmark := "/" + benchProcs
 						descMetric := "/" + metric
@@ -171,31 +173,19 @@ func perfGraphHandler(w http.ResponseWriter, r *http.Request) {
 						}
 						desc := fmt.Sprintf("%v%v%v", descBuilder, descBenchmark, descMetric)[1:]
 						hh := make([]string, commitsToDisplay)
+						ann := make([]string, commitsToDisplay)
 						valf := make([]float64, commitsToDisplay)
 						cert := make([]bool, commitsToDisplay)
 						firstval := uint64(0)
 						lastval := uint64(0)
-						lastval0 := uint64(0)
 						for i, v := range vv {
 							cert[i] = true
 							if v == 0 {
 								if lastval == 0 {
 									continue
 								}
-								nextval := uint64(0)
-								nextidx := 0
-								for i2, v2 := range vv[i+1:] {
-									if v2 != 0 {
-										nextval = v2
-										nextidx = i + i2 + 1
-										break
-									}
-								}
-								if nextval == 0 {
-									continue
-								}
 								cert[i] = false
-								v = lastval + uint64(int64(nextval-lastval)/int64(nextidx-i+1))
+								v = lastval
 							}
 							if firstval == 0 {
 								firstval = v
@@ -203,21 +193,22 @@ func perfGraphHandler(w http.ResponseWriter, r *http.Request) {
 							valf[i] = float64(v) / float64(firstval)
 							if cert[i] {
 								d := ""
-								if lastval0 != 0 {
-									d = fmt.Sprintf(" (%.02f%%)", perfDiff(lastval0, v))
+								if lastval != 0 {
+									diff := perfDiff(lastval, v)
+									d = fmt.Sprintf(" (%+.02f%%)", diff)
+									if !isNoise(diff, noise) {
+										ann[i] = fmt.Sprintf("%+.02f%%", diff)
+									}
 								}
 								hh[i] = fmt.Sprintf("%v%v", v, d)
-
 							} else {
 								hh[i] = "NO DATA"
 							}
 							lastval = v
-							if cert[i] {
-								lastval0 = v
-							}
 						}
 						vals = append(vals, valf)
 						hints = append(hints, hh)
+						annotations = append(annotations, ann)
 						certainty = append(certainty, cert)
 						headers = append(headers, desc)
 					}
@@ -237,7 +228,7 @@ func perfGraphHandler(w http.ResponseWriter, r *http.Request) {
 			c := perfGraphCommit{Id: idx, Name: fmt.Sprintf("%v (%v)", com.Desc, com.Time.Format("Jan 2, 2006 1:04"))}
 			idx++
 			for j := range vals {
-				c.Vals = append(c.Vals, perfGraphValue{float64(vals[j][i]), certainty[j][i], hints[j][i]})
+				c.Vals = append(c.Vals, perfGraphValue{float64(vals[j][i]), certainty[j][i], hints[j][i], annotations[j][i]})
 			}
 			commits = append(commits, c)
 		}
@@ -275,4 +266,5 @@ type perfGraphValue struct {
 	Val       float64
 	Certainty bool
 	Hint      string
+	Ann       string
 }

dashboard/app/build/perf_graph.html

@@ -20,13 +20,14 @@
           data.addColumn({type: 'number', label: '{{.}}'});
           data.addColumn({type: 'boolean', role: 'certainty'});
           data.addColumn({type: 'string', role: 'tooltip'});
+          data.addColumn({type: 'string', role: 'annotation'});
         {{end}}
         data.addRows([
         {{range $.Commits}}
           [ {{.Id}}, 1, "{{.Name}}",
             {{range .Vals}}
               {{if .Val}}
-                {{.Val}}, {{.Certainty}}, '{{.Hint}}',
+                {{.Val}}, {{.Certainty}}, '{{.Hint}}', '{{.Ann}}',
               {{else}}
                 ,,,
               {{end}}