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))