Fix graph not rendering flat lines

Author: rs

data/point.go

@@ -58,7 +58,7 @@ func (p *Points) push(name string, value float64, counter bool) {
 	d := p.getLocked(name)
 	if counter {
 		var diff float64
-		if last := p.last[name]; last > 0 {
+		if last := p.last[name]; last > 0 && last < diff {
 			diff = value - last
 		}
 		p.last[name] = value

graph/dash.go

@@ -16,7 +16,7 @@ type Dash struct {
 }
 
 // Render generates a PNG with all graphs stacked.
-func (d Dash) Render(w io.Writer, width, height int) {
+func (d Dash) Render(w io.Writer, width, height int) error {
 	graphs := make([]chart.Chart, 0, len(d.Specs))
 	for _, spec := range d.Specs {
 		graphs = append(graphs, New(spec, d.Data, width, height/len(d.Specs)))
@@ -25,11 +25,13 @@ func (d Dash) Render(w io.Writer, width, height int) {
 	var top int
 	for _, graph := range graphs {
 		iw := &chart.ImageWriter{}
-		graph.Render(chart.PNG, iw)
+		if err := graph.Render(chart.PNG, iw); err != nil {
+			return err
+		}
 		img, _ := iw.Image()
 		r := image.Rectangle{image.Point{0, top}, image.Point{width, top + graph.Height}}
 		top += graph.Height
 		draw.Draw(canvas, r, img, image.Point{0, 0}, draw.Src)
 	}
-	png.Encode(w, canvas)
+	return png.Encode(w, canvas)
 }

graph/graph.go

@@ -42,8 +42,10 @@ func New(spec data.Spec, dp *data.Points, width, height int) chart.Chart {
 }
 
 func newChart(series []chart.Series, markers []chart.GridLine, width, height int) chart.Chart {
+	var min, max float64 = math.MaxFloat64, -math.MaxFloat64
 	for i, s := range series {
 		if s, ok := s.(chart.ContinuousSeries); ok {
+			min, max = minMax(s.YValues, min, max)
 			s.XValues = seq.Range(0, float64(len(s.YValues)-1))
 			c := chart.GetAlternateColor(i + 4)
 			s.Style = chart.Style{
@@ -74,6 +76,15 @@ func newChart(series []chart.Series, markers []chart.GridLine, width, height int
 		},
 		Series: series,
 	}
+	if min == max {
+		// By default, go-chart will fail to render a flat line as the range will be NaN.
+		// Define a manual range in such case.
+		// See https://github.com/wcharczuk/go-chart/issues/31
+		graph.YAxis.Range = &chart.ContinuousRange{
+			Min: min - 0.05,
+			Max: max + 0.05,
+		}
+	}
 	if len(markers) > 0 {
 		graph.Background.Padding.Bottom = 0 // compensate transparent tick space
 		graph.XAxis = chart.XAxis{
@@ -101,6 +112,19 @@ func newChart(series []chart.Series, markers []chart.GridLine, width, height int
 	return graph
 }
 
+func minMax(values []float64, curMin, curMax float64) (min, max float64) {
+	min, max = curMin, curMax
+	for _, value := range values {
+		if value < min {
+			min = value
+		}
+		if value > max {
+			max = value
+		}
+	}
+	return
+}
+
 func siValueFormater(v interface{}) string {
 	value, prefix := humanize.ComputeSI(v.(float64))
 	value = float64(int(value*100)) / 100

main.go

@@ -151,5 +151,7 @@ func render(dash graph.Dash, rows int) {
 	// Use iTerm2 image display feature.
 	term := &osc.ImageWriter{}
 	defer term.Close()
-	dash.Render(term, width, height)
+	if err := dash.Render(term, width, height); err != nil {
+		fatal(fmt.Sprintf("cannot render graph: %v", err.Error()))
+	}
 }