Layers

Using a scatterplot probably isn't the best for visualizing change over time. Instead, let's tell ggplot to visualize the data as a line plot:

ggplot(data = gapminder, mapping = aes(x=year, y=lifeExp, color=continent)) +
geom_line()


Instead of adding a geom_point layer, we've added a geom_line layer.

However, the result doesn't look quite as we might have expected: it seems to be jumping around a lot in each continent. Let's try to separate the data by country, plotting one line for each country:

ggplot(data = gapminder, mapping = aes(x=year, y=lifeExp, by=country, color=continent)) +
geom_line()


We've added the by aesthetic, which tells ggplot to draw a line for each country.

But what if we want to visualize both lines and points on the plot? We can add another layer to the plot:

ggplot(data = gapminder, mapping = aes(x=year, y=lifeExp, by=country, color=continent)) +
geom_line() + geom_point()


It's important to note that each layer is drawn on top of the previous layer. In this example, the points have been drawn on top of the lines. Here's a demonstration:

ggplot(data = gapminder, mapping = aes(x=year, y=lifeExp, by=country)) +
geom_line(mapping = aes(color=continent)) + geom_point()


In this example, the aesthetic mapping of color has been moved from the global plot options in ggplot to the geom_line layer so it no longer applies to the points. Now we can clearly see that the points are drawn on top of the lines.


Tip: Setting an aesthetic to a value instead of a mapping

So far, we've seen how to use an aesthetic (such as color) as a mapping to a variable in the data. For example, when we use geom_line(mapping = aes(color=continent)), ggplot will give a different color to each continent. But what if we want to change the color of all lines to blue? You may think that geom_line(mapping = aes(color="blue")) should work, but it doesn't. Since we don't want to create a mapping to a specific variable, we can move the color specification outside of the aes() function, like this: geom_line(color="blue").