fix: cap telemetry chart Y-axis when outliers distort scale (#686)

Turbostat and sar telemetry data occasionally contains single readings
that are orders of magnitude too high, compressing useful data into a
tiny band at the bottom of HTML charts. Add percentile-based outlier
detection that sets a hard Y-axis max when the actual maximum exceeds
P99 by more than 50%, keeping charts readable while preserving outlier
values in tooltips. JSON and XLSX outputs are unaffected.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: harp-intel

cmd/telemetry/telemetry_renderers.go

@@ -15,11 +15,38 @@ import (
 	"strings"
 )
 
+// computeAxisMax examines all datasets and returns a Y-axis hard max string.
+// If outliers are detected (actual max > P99 * 1.5), it returns a value slightly
+// above P99. Otherwise it returns "" (no constraint, use auto-scale).
+func computeAxisMax(data [][]float64) string {
+	var all []float64
+	for _, dataset := range data {
+		all = append(all, dataset...)
+	}
+	if len(all) < 4 {
+		return ""
+	}
+	sorted := make([]float64, len(all))
+	copy(sorted, all)
+	slices.Sort(sorted)
+	p99Idx := int(float64(len(sorted)-1) * 0.99)
+	p99 := sorted[p99Idx]
+	actualMax := sorted[len(sorted)-1]
+	if p99 > 0 && actualMax > p99*1.5 {
+		return fmt.Sprintf("%f", p99*1.1)
+	}
+	return ""
+}
+
 func telemetryTableHTMLRenderer(tableValues table.TableValues, data [][]float64, datasetNames []string, chartConfig report.ChartTemplateStruct, datasetHiddenFlags []bool) string {
 	if len(tableValues.Fields) == 0 {
 		slog.Error("no fields in table", slog.String("table", tableValues.Name))
 		return ""
 	}
+	// Auto-detect outliers and set hard Y-axis max for auto-scaled charts
+	if chartConfig.YaxisMax == "" && chartConfig.SuggestedMax == "0" {
+		chartConfig.YaxisMax = computeAxisMax(data)
+	}
 	tsFieldIdx := 0
 	var timestamps []string
 	for i := range tableValues.Fields[0].Values {

cmd/telemetry/telemetry_renderers_test.go

@@ -0,0 +1,92 @@
+// Copyright (C) 2021-2025 Intel Corporation
+// SPDX-License-Identifier: BSD-3-Clause
+
+package telemetry
+
+import (
+	"fmt"
+	"testing"
+)
+
+func TestComputeAxisMax(t *testing.T) {
+	tests := []struct {
+		name      string
+		data      [][]float64
+		wantEmpty bool // true means expect "" (no constraint)
+		wantAbove float64
+		wantBelow float64
+	}{
+		{
+			name:      "normal data, no outliers",
+			data:      [][]float64{{10, 12, 11, 13, 10, 12, 11, 14, 10, 13}},
+			wantEmpty: true,
+		},
+		{
+			name:      "single extreme outlier",
+			data:      [][]float64{{10, 12, 11, 13, 10, 12, 11, 14, 10, 10000}},
+			wantEmpty: false,
+			wantAbove: 13,
+			wantBelow: 10000,
+		},
+		{
+			name:      "all identical values",
+			data:      [][]float64{{5, 5, 5, 5, 5, 5, 5, 5, 5, 5}},
+			wantEmpty: true,
+		},
+		{
+			name:      "too few data points",
+			data:      [][]float64{{10, 20, 30}},
+			wantEmpty: true,
+		},
+		{
+			name:      "empty data",
+			data:      [][]float64{},
+			wantEmpty: true,
+		},
+		{
+			name:      "multiple datasets, one with outlier",
+			data:      [][]float64{{10, 12, 11, 13, 10}, {11, 14, 10, 13, 50000}},
+			wantEmpty: false,
+			wantAbove: 13,
+			wantBelow: 50000,
+		},
+		{
+			name:      "gradual increase, no outlier",
+			data:      [][]float64{{1, 2, 3, 4, 5, 6, 7, 8, 9, 10}},
+			wantEmpty: true,
+		},
+		{
+			name:      "all zeros",
+			data:      [][]float64{{0, 0, 0, 0, 0}},
+			wantEmpty: true,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := computeAxisMax(tt.data)
+			if tt.wantEmpty {
+				if got != "" {
+					t.Errorf("computeAxisMax() = %q, want empty string", got)
+				}
+				return
+			}
+			if got == "" {
+				t.Errorf("computeAxisMax() = empty string, want a constraining value")
+				return
+			}
+			// Parse and check bounds
+			var val float64
+			n, err := fmt.Sscanf(got, "%f", &val)
+			if err != nil || n != 1 {
+				t.Errorf("computeAxisMax() = %q, could not parse as float", got)
+				return
+			}
+			if val <= tt.wantAbove {
+				t.Errorf("computeAxisMax() = %f, want > %f", val, tt.wantAbove)
+			}
+			if val >= tt.wantBelow {
+				t.Errorf("computeAxisMax() = %f, want < %f", val, tt.wantBelow)
+			}
+		})
+	}
+}

internal/report/render_html.go

@@ -448,7 +448,8 @@ new Chart(document.getElementById('{{.ID}}'), {
                     display: true
                 },
 				suggestedMin: {{.SuggestedMin}},
-				suggestedMax: {{.SuggestedMax}},
+				suggestedMax: {{.SuggestedMax}},{{if .YaxisMax}}
+				max: {{.YaxisMax}},{{end}}
             }
         },
         plugins: {
@@ -525,7 +526,8 @@ new Chart(document.getElementById('{{.ID}}'), {
                     display: true
                 },
 				suggestedMin: {{.SuggestedMin}},
-				suggestedMax: {{.SuggestedMax}},
+				suggestedMax: {{.SuggestedMax}},{{if .YaxisMax}}
+				max: {{.YaxisMax}},{{end}}
             }
         },
         plugins: {
@@ -590,6 +592,7 @@ type ChartTemplateStruct struct {
 	AspectRatio   string
 	SuggestedMin  string
 	SuggestedMax  string
+	YaxisMax      string // hard max for Y-axis; "" means no constraint
 }
 
 // CreateFieldNameWithDescription creates HTML for a field name with optional description tooltip