From 7ae42ee2e13974870ad0b459cf5819d034bfc260 Mon Sep 17 00:00:00 2001
From: Will Charczuk <will@blendlabs.com>
Date: Fri, 13 Jan 2017 14:07:42 -0800
Subject: [PATCH] handle zero values with error.

---
 pie_chart.go      | 14 +++++++++++---
 pie_chart_test.go | 38 ++++++++++++++++++++++++++++++++++++++
 value.go          | 21 ++++++++++++++-------
 3 files changed, 63 insertions(+), 10 deletions(-)

diff --git a/pie_chart.go b/pie_chart.go
index 720f61d..b2be1a6 100644
--- a/pie_chart.go
+++ b/pie_chart.go
@@ -2,6 +2,7 @@ package chart
 
 import (
 	"errors"
+	"fmt"
 	"io"
 
 	"github.com/golang/freetype/truetype"
@@ -88,7 +89,10 @@ func (pc PieChart) Render(rp RendererProvider, w io.Writer) error {
 	pc.drawBackground(r)
 	pc.drawCanvas(r, canvasBox)
 
-	finalValues := pc.finalizeValues(pc.Values)
+	finalValues, err := pc.finalizeValues(pc.Values)
+	if err != nil {
+		return err
+	}
 	pc.drawSlices(r, canvasBox, finalValues)
 	pc.drawTitle(r)
 	for _, a := range pc.Elements {
@@ -158,8 +162,12 @@ func (pc PieChart) drawSlices(r Renderer, canvasBox Box, values []Value) {
 	}
 }
 
-func (pc PieChart) finalizeValues(values []Value) []Value {
-	return Values(values).Normalize()
+func (pc PieChart) finalizeValues(values []Value) ([]Value, error) {
+	finalValues := Values(values).Normalize()
+	if len(finalValues) == 0 {
+		return nil, fmt.Errorf("pie chart must contain at least (1) non-zero value")
+	}
+	return finalValues, nil
 }
 
 func (pc PieChart) getDefaultCanvasBox() Box {
diff --git a/pie_chart_test.go b/pie_chart_test.go
index 2f536cd..788cbe2 100644
--- a/pie_chart_test.go
+++ b/pie_chart_test.go
@@ -29,3 +29,41 @@ func TestPieChart(t *testing.T) {
 	pie.Render(PNG, b)
 	assert.NotZero(b.Len())
 }
+
+func TestPieChartDropsZeroValues(t *testing.T) {
+	assert := assert.New(t)
+
+	pie := PieChart{
+		Canvas: Style{
+			FillColor: ColorLightGray,
+		},
+		Values: []Value{
+			{Value: 5, Label: "Blue"},
+			{Value: 5, Label: "Green"},
+			{Value: 0, Label: "Gray"},
+		},
+	}
+
+	b := bytes.NewBuffer([]byte{})
+	err := pie.Render(PNG, b)
+	assert.Nil(err)
+}
+
+func TestPieChartAllZeroValues(t *testing.T) {
+	assert := assert.New(t)
+
+	pie := PieChart{
+		Canvas: Style{
+			FillColor: ColorLightGray,
+		},
+		Values: []Value{
+			{Value: 0, Label: "Blue"},
+			{Value: 0, Label: "Green"},
+			{Value: 0, Label: "Gray"},
+		},
+	}
+
+	b := bytes.NewBuffer([]byte{})
+	err := pie.Render(PNG, b)
+	assert.NotNil(err)
+}
diff --git a/value.go b/value.go
index 8a1da0d..0346c9a 100644
--- a/value.go
+++ b/value.go
@@ -26,13 +26,20 @@ func (vs Values) ValuesNormalized() []float64 {
 
 // Normalize returns the values normalized.
 func (vs Values) Normalize() []Value {
-	output := make([]Value, len(vs))
-	total := Math.Sum(vs.Values()...)
-	for index, v := range vs {
-		output[index] = Value{
-			Style: v.Style,
-			Label: v.Label,
-			Value: Math.RoundDown(v.Value/total, 0.0001),
+	var output []Value
+	var total float64
+
+	for _, v := range vs {
+		total += v.Value
+	}
+
+	for _, v := range vs {
+		if v.Value > 0 {
+			output = append(output, Value{
+				Style: v.Style,
+				Label: v.Label,
+				Value: Math.RoundDown(v.Value/total, 0.0001),
+			})
 		}
 	}
 	return output