plot: multi-line title is leaking into data canvas area

[sbinet]

this simple plot script:

//go:build ignore
// +build ignore

package main

import (
	"flag"
	"fmt"
	"image/color"
	"log"

	"gonum.org/v1/plot"
	"gonum.org/v1/plot/plotter"
	"gonum.org/v1/plot/vg"
)

func main() {
	flag.Parse()

	sha := flag.Arg(0)

	var err error
	p, err := plot.New()
	if err != nil {
		log.Fatal(err)
	}
	p.Title.Text = "T\nmy title\nUU"
	p.X.Min = 0
	p.X.Max = 10
	p.Y.Min = 0
	p.Y.Max = 10

	f1 := plotter.NewFunction(func(x float64) float64 { return 5 })
	f1.LineStyle.Color = color.RGBA{R: 255, A: 255}

	f2 := plotter.NewFunction(func(x float64) float64 { return 6 })
	f2.LineStyle.Color = color.RGBA{B: 255, A: 255}

	p.Add(plotter.NewGlyphBoxes())
	p.Add(f1, f2)
	p.Add(plotter.NewGrid())

	p.Legend.Add("f1", f1)
	p.Legend.Add("f2", f2)
	p.Legend.Top = true

	fname := fmt.Sprintf("box-%s.png", sha)
	fmt.Printf("saving to %q...\n", fname)
	err = p.Save(20*vg.Centimeter, 15*vg.Centimeter, fname)
	if err != nil {
		log.Fatalf("error: %+v", err)
	}
}

produces (with v0.9.0):

this is coming from the change of semantics of text.Handler.Box in aa25907 that's supposed to only deal with non-multiline text.
whereas:
https://github.com/gonum/plot/blob/v0.9.0/plot.go#L150..L156

	if p.Title.Text != "" {
		descent := p.Title.TextStyle.FontExtents().Descent
		c.FillText(p.Title.TextStyle, vg.Point{X: c.Center().X, Y: c.Max.Y + descent}, p.Title.Text)
		_, h, d := p.Title.TextStyle.Handler.Box(p.Title.Text, p.Title.TextStyle.Font)
		c.Max.Y -= h + d
		c.Max.Y -= p.Title.Padding
	}

clearly doesn't handle the multi-line case.

we could introduce a Box function in text:

package text

// Box returns the bounding box of the given possibly-multiline text where:
//  - width is the horizontal space from the origin of the widest line,
//  - height is the vertical space, from the bottom of the descent up to the ascent,
//    including any needed linegaps.
func Box(hdlr Handler, txt string, fnt Font) (width, height Length) {
        var (
                lines   = hdlr.Lines(txt)
                e       = hdlr.FontExtents(fnt)
                linegap = (e.Height - e.Ascent - e.Descent)
        )
        for i, line := range lines {
                ww, hh, dd := hdlr.Box(line, fnt)
                if ww > width {
                        width = ww
                }
                height += hh + dd
                if i > 0 {
                        height += linegap
                }
        }
        return width, height
}

(which is more or less the implementation of text.Style.box, that method could then just forward to that new top-level function.)