With thanks to this file by Jenny Bryan.

First basic plot

suppressMessages(library("ggplot2"))
suppressMessages(library("gapminder"))
gDat <- gapminder

p <- ggplot(gDat, aes(x = gdpPercap, y = lifeExp)) # just initializes
p + geom_point()  # # scatterplot

# idem:  p + layer(geom = "point")

In logs

#' log transformation ... quick and dirty
ggplot(gDat, aes(x = log10(gdpPercap), y = lifeExp)) + geom_point()

#' a better way to log transform
p + geom_point() + scale_x_log10()

#' let's make that stick
p <- p + scale_x_log10()
#' common workflow: gradually build up the plot you want  
#' re-define the object 'p' as you develop "keeper" commands

By Continent

#' convey continent by color: MAP continent variable to aesthetic color
p + geom_point(aes(color = continent))

ggplot(gDat, aes(x = gdpPercap, y = lifeExp, color = continent)) +
    geom_point() + scale_x_log10() # in full detail, up to now

#' address overplotting: SET alpha transparency and size to a value
p + geom_point(alpha = (1/3), size = 3)