func NewLegend() (Legend, error) {
	font, err := vg.MakeFont(DefaultFont, vg.Points(12))
	if err != nil {
		return Legend{}, err
	}
	return Legend{
		ThumbnailWidth: vg.Points(20),
		TextStyle:      draw.TextStyle{Font: font},
	}, nil
}
NewLegend returns a legend with the default
parameter settings.
package plot
import (
	"math"
	"gonum.org/v1/plot/vg"
	"gonum.org/v1/plot/vg/draw"
)
type Legend struct {
	draw.TextStyle
	Padding vg.Length
	Top, Left bool
	XOffs, YOffs vg.Length
	ThumbnailWidth vg.Length
	entries []legendEntry
}
type legendEntry struct {
	text string
	thumbs []Thumbnailer
}
type Thumbnailer interface {
	Thumbnail(c *draw.Canvas)
}