plot/plotter: Grid plotter adds a spurious line

when debugging the Example_logScale output, I noticed that when changing the size of the output plot from 10x10 vg.Centimeter to 5x5, I got the attached figure 1).

Putting " " as a title shows the problem better figure 2).

here is the code:

// Example_logScale shows how to create a plot with a log-scale on the Y-axis.
func Example_logScale() {
	p, err := plot.New()
	if err != nil {
		log.Fatal(err)
	}

	p.Title.Text = " "
	p.Y.Scale = plot.LogScale{}
	p.Y.Tick.Marker = plot.LogTicks{}
	p.X.Label.Text = "x"
	p.Y.Label.Text = "f(x)"

	f := plotter.NewFunction(math.Exp)
	f.Color = color.RGBA{R: 255, A: 255}

	p.Add(f, plotter.NewGrid())
	p.Legend.Add("exp(x)", f)

	p.X.Min = 0.1
	p.X.Max = 10
	p.Y.Min = math.Exp(p.X.Min)
	p.Y.Max = math.Exp(p.X.Max)

	err = p.Save(5*vg.Centimeter, 5*vg.Centimeter, "testdata/logscale.png")
	if err != nil {
		log.Panic(err)
	}
}

But when setting the title to an empty string "", I get the figure 3).

• figure 1)

• figure 2)

• figure 3)