plot: Align doesn't exactly align axes

[sbinet]

as can be noticed from the produced plot in #732, (even with the 1:1 ratio), axes are not exactly aligned between plots (Y-axis for top-left/bottom-left, X-axis for bottom-left/bottom-right):

I've tested that with release up to v0.7.0 (so it's not a fall out from the "fonts revamp" work.)

[sbinet]

ha! it would seem the misalignment is coming from the glyphs being used for the scatter plot.
playing with the circle radius:

so it's probably some interplay between plot.Plot.DataCanvas and plot.GlyphBoxer
@ctessum any insight?

[dblueman]

The same axis slight misalignment can be seen when adjusting the heatmap example for a PDF canvas:

package main

import (
	"fmt"
	"math"
	"os"

	"gonum.org/v1/gonum/mat"
	"gonum.org/v1/plot"
	"gonum.org/v1/plot/palette"
	"gonum.org/v1/plot/plotter"
	"gonum.org/v1/plot/vg"
	"gonum.org/v1/plot/vg/draw"
	"gonum.org/v1/plot/vg/vgpdf"
)

type offsetUnitGrid struct {
	XOffset, YOffset float64
	Data             mat.Matrix
}

type integerTicks struct{}

func (g offsetUnitGrid) Dims() (c, r int) { r, c = g.Data.Dims(); return c, r }

func (g offsetUnitGrid) Z(c, r int) float64 { return g.Data.At(r, c) }

func (g offsetUnitGrid) X(c int) float64 {
	_, n := g.Data.Dims()
	if c < 0 || c >= n {
		panic("range check")
	}
	return float64(c) + g.XOffset
}

func (g offsetUnitGrid) Y(r int) float64 {
	m, _ := g.Data.Dims()
	if r < 0 || r >= m {
		panic("range check")
	}
	return float64(r) + g.YOffset
}

func (integerTicks) Ticks(min, max float64) []plot.Tick {
	var t []plot.Tick
	for i := math.Trunc(min); i <= max; i++ {
		t = append(t, plot.Tick{Value: i, Label: fmt.Sprint(i)})
	}
	return t
}

func top() error {
	m := offsetUnitGrid{
		XOffset: -2,
		YOffset: -1,
		Data: mat.NewDense(3, 4, []float64{
			1, 2, 3, 4,
			5, 6, 7, 8,
			9, 10, 11, 12,
		})}
	pal := palette.Heat(12, 1)
	h := plotter.NewHeatMap(m, pal)

	p := plot.New()
	p.Title.Text = "Heat map"

	p.X.Tick.Marker = integerTicks{}
	p.Y.Tick.Marker = integerTicks{}

	p.Add(h)

	l := plot.NewLegend()
	thumbs := plotter.PaletteThumbnailers(pal)

	for i := range len(thumbs) {
		l.Add(fmt.Sprint(i), thumbs[i])
	}

	p.X.Padding = 0
	p.Y.Padding = 0
	p.X.Max = 1.5
	p.Y.Max = 1.5

	img := vgpdf.New(800, 600)
	dc := draw.New(img)

	l.Top = true

	r := l.Rectangle(dc)
	legendWidth := r.Max.X - r.Min.X
	l.YOffs = -p.Title.TextStyle.FontExtents().Height

	l.Draw(dc)
	// allow space for legend
	dc = draw.Crop(dc, 0, -legendWidth-vg.Millimeter, 0, 0)
	p.Draw(dc)
	w, err := os.Create("heatmap.pdf")
	if err != nil {
		return err
	}
	defer w.Close()

	_, err = img.WriteTo(w)
	if err != nil {
		return err
	}

	return nil
}

func main() {
	err := top()
	if err != nil {
		fmt.Println(err)
		os.Exit(1)
	}
}

Bottom left corner of rendered PDF: