Scale area instead of radius, for size.
scale_size_area is used, the default behavior
is to scale the area of points to be proportional to the
scale_size_area(..., max_size = 6)
- Other arguments passed on to
continuous_scaleto control name, limits, breaks, labels and so forth.
- Size of largest points.
Note that this controls the size scale, so it will also control the thickness of lines. Line thickness will be proportional to the square root of the value, which is probably undesirable in most cases.