Define types plotter.XY and plotter.XYZ for append

Created by: chmike

What are you trying to do?

Creating a plotter.XYs value by appending its points with append because the total number of points is unknown when I start.

What did you do?

points := make(plotter.XYs, 0, 1024)
for {
    ...
    points = append(points, struct{ X, Y float64 }{x, y})
}

What did you expect to happen?

I expect I could write the following which is more readable and intuitive.

points = append(points, plotter.XY{x, y})

What version of Go and Gonum/plot are you using?

Current

Does this issue reproduce with the current master?

Yes