Geometric Objects Demo

1. What geom would you use to draw a line chart? A boxplot? A histogram? An area chart?

• line chart: geom_line()
• boxplot: geom_boxplot()
• histogram: geom_histogram()
• area chart: geom_area()

2. Run this code in your head and predict what the output will look like. Then, run the code in R and check your predictions.

ggplot(data = mpg, mapping = aes(x = displ, y = hwy, colour = drv)) + geom_point() + geom_smooth(se = FALSE)

This code produces a scatter plot with displ on the x-axis, hwy on the y-axis, and the points colored by drv. There will be a smooth line, without standard errors, fit through each drv group.

ggplot(data = mpg, mapping = aes(x = displ, y = hwy, colour = drv)) + geom_point() + geom_smooth(se = FALSE)

3.What does show.legend = FALSE do? What happens if you remove it? Why do you think I used it earlier in the lecture?

The theme option show.legend = FALSE hides the legend box.

Consider this example earlier in the chapter.

ggplot(data = mpg) +
  geom_smooth(
    mapping = aes(x = displ, y = hwy, colour = drv),
    show.legend = FALSE
  )

In that plot, there is no legend. Removing the show.legend argument or setting show.legend = TRUE will result in the plot having a legend displaying the mapping between colors and drv.

ggplot(data = mpg) +
  geom_smooth(mapping = aes(x = displ, y = hwy, colour = drv))

In the lecture, the legend is suppressed because with three plots, adding a legend to only the last plot would make the sizes of plots different. Different sized plots would make it more difficult to see how arguments change the appearance of the plots. The purpose of those plots is to show the difference between no groups, using a group aesthetic, and using a color aesthetic, which creates implicit groups. In that example, the legend isn’t necessary since looking up the values associated with each color isn’t necessary to make that point.

4. What does the se argument to geom_smooth() do?

It adds standard error bands to the lines.

ggplot(data = mpg, mapping = aes(x = displ, y = hwy, colour = drv)) + geom_point() + geom_smooth(se = TRUE)

By default se = TRUE:

ggplot(data = mpg, mapping = aes(x = displ, y = hwy, colour = drv)) + geom_point() +  geom_smooth()

5. Will these two graphs look different? Why/why not?

ggplot(data = mpg, mapping = aes(x = displ, y = hwy)) +
  geom_point() +
  geom_smooth()

ggplot() +
  geom_point(data = mpg, mapping = aes(x = displ, y = hwy)) +
  geom_smooth(data = mpg, mapping = aes(x = displ, y = hwy))

No. Because both geom_point() and geom_smooth() will use the same data and mappings. They will inherit those options from the ggplot() object, so the mappings don’t need to specified again.

6. Recreate the R code necessary to generate the following graphs.

The following code will generate those plots.

ggplot(mpg, aes(x = displ, y = hwy)) +
  geom_point() +
  geom_smooth(se = FALSE)

ggplot(mpg, aes(x = displ, y = hwy)) +
  geom_smooth(mapping = aes(group = drv), se = FALSE) +
  geom_point()

ggplot(mpg, aes(x = displ, y = hwy, colour = drv)) +
  geom_point() +
  geom_smooth(se = FALSE)

ggplot(mpg, aes(x = displ, y = hwy)) +
  geom_point(aes(colour = drv)) +
  geom_smooth(se = FALSE)

ggplot(mpg, aes(x = displ, y = hwy)) +
  geom_point(aes(colour = drv)) +
  geom_smooth(aes(linetype = drv), se = FALSE)

ggplot(mpg, aes(x = displ, y = hwy)) +
  geom_point(size = 4, color = "white") +
  geom_point(aes(colour = drv))