Adding NewXYs func to plotter.go will be more convenient

[linw1995]

Adding this function will be more convenient.

• Before adding, you create XYs like this

x := plotter.Values{0.1, 0.2, 0.3, 0.4}
y := plotter.Values{0.1, 0.2, 0.3, 0.4}
xys := make(plotter.XYs, x.Len())
for i := range xys {
	xys[i].X, xys[i].Y = x.Value(i), y.Value(i)
}

• After adding, you can do more pretty

xys := plotter.NewXYs(x, y)

An Example of func NewXYs

// NewXYs returns an XYs that is a combination of the x, y Valuer,
// or an error if x's length is not equal to y's.
func NewXYs(x, y Valuer) XYs {
	if x.Len() != y.Len() {
		panic(fmt.Errorf("Len mismatch, %d != %d", x.Len(), y.Len()))
	}
	xys := make(XYs, x.Len())
	for i := range xys {
		xys[i].X, xys[i].Y = x.Value(i), y.Value(i)
	}
	return xys
}

[sbinet]

a different take on #284 and #315