How to add point size to a Gadfly scatter plot based on a variable

[Brinkhuis]

@evanfields

p = plot(
    mpg,
    x = :horsepower,
    y = :mpg,
    color = :origin,
    size = :weight,
    alpha = [0.5],
    Geom.point,
    Theme(background_color = "white"),
    Scale.size_area(maxvalue=maximum(mpg.weight)),
)

Thanks for your detailed answer, highly appreciated!