# Bar chart example
c <- ggplot(mtcars, aes(factor(cyl)))
# Default plotting
c + geom_bar()
# To change the interior colouring use fill aesthetic
c + geom_bar(fill = "red")
# Compare with the colour aesthetic which changes just the bar outline
c + geom_bar(colour = "red")
# Combining both, you can see the changes more clearly
c + geom_bar(fill = "white", colour = "red")

# The aesthetic fill also takes different colouring scales
# setting fill equal to a factor variable uses a discrete colour scale
k <- ggplot(mtcars, aes(factor(cyl), fill = factor(vs)))
k + geom_bar()

# Fill aesthetic can also be used with a continuous variable
m <- ggplot(faithfuld, aes(waiting, eruptions))
m + geom_raster()
m + geom_raster(aes(fill = density))

# Some geoms don't use both aesthetics (i.e. geom_point or geom_line)
b <- ggplot(economics, aes(x = date, y = unemploy))
b + geom_line()
b + geom_line(colour = "green")
b + geom_point()
b + geom_point(colour = "red")

# For large datasets with overplotting the alpha
# aesthetic will make the points more transparent
df <- data.frame(x = rnorm(5000), y = rnorm(5000))
h  <- ggplot(df, aes(x,y))
h + geom_point()
h + geom_point(alpha = 0.5)
h + geom_point(alpha = 1/10)

# Alpha can also be used to add shading
j <- b + geom_line()
j
yrng <- range(economics$unemploy)
j <- j + geom_rect(aes(NULL, NULL, xmin = start, xmax = end, fill = party),
ymin = yrng[1], ymax = yrng[2], data = presidential)
j
j + scale_fill_manual(values = alpha(c("blue", "red"), .3))