Unable to generate graph

[wengooooo]

Reproduction code

x1 := time.Now().Add(10 * time.Second)
	x2 := time.Now().Add(15 * time.Second)
	x3 := time.Now().Add(20 * time.Second)
	graph := chart.Chart{
		Series: []chart.Series{
			chart.TimeSeries{
				XValues: []time.Time{x1, x2, x3},
				YValues: []float64{1.0, 1.0, 1.0},
			},
			chart.AnnotationSeries{
				Annotations: []chart.Value2{
					{XValue: chart.TimeToFloat64(x1), YValue: 1.0, Label: "first"},
					{XValue: chart.TimeToFloat64(x2), YValue: 1.0, Label: "second"},
					{XValue: chart.TimeToFloat64(x3), YValue: 1.0, Label: "last"},
				},
			},
		},
	}

	f, _ := os.Create("output.png")
	defer f.Close()
	fmt.Println(graph.Render(chart.PNG, f))