R has become very popular not just because of its data visualization capabilities but also its ability to combine data visualizations with statistical information. The statistics layer helps us represent statistical information about the data, such as mean and variance, to help in understanding the data.
We earlier learned about geoms which stand for "geometric objects." These are the core elements that you see on the plot, and includes objects such as points, lines, areas, and curves. Stats stand for "statistical transformations." These help us summarize the data in different ways such as counting observations, creating a linear regression line that best fits the data, or adding a confidence interval to the regression line.
All geoms have a default stat. For example, when we create a bar chart using geom_bars, the default stat is stat_count, which counts the number of rows to create the bar chart. You can add statistics layers to the plot using the stat_ functions.
Stat
Description
bin
Divide continuous range into bins, and count number of points in each
boxplot
Compute statistics necessary for boxplot
contour
Calculate contour lines
density
Compute 1d density estimate
identity
Identity transformation,f(x)=x
jitter
Jitter values by adding small random value
qq
Calculate values for quantile-quantile plot
quantile
Quantile regression
smooth
Smoothed conditional mean of y given x
summary
Aggregate values of y for given x
unique
Remove duplicated observations
Let's look at some of these statistics.
Smoothing
One of the most common statistical transformations we add to our plots is a smoothing line. When we plot a scatter chart, we can add a smoothing line using the geom_smooth(), which in turn uses stat_smooth to plot the smooth curve.
Let's use our stock_prices dataset and create a scatter plot with AAPL prices on x-axis and GOOGL prices on y-axis. We will add a smooth line to the scatter plot (LOESS is the default) with geom_smooth().
We can modify various aspects of stat_smooth. For example, span can be used to control the amount of smoothing for the default loess smoother. Smaller numbers produce wigglier lines, larger numbers produce smoother lines.
In the following example, we set the span to 0.3 and also change the line color to red.
