Outliers are a common issue in data analysis. Although an exact definition of outliers doesn't exist, we know that outliers can influence means and regression results. Outliers are values that are anomalous. Usually, outliers are caused by a measurement error, but the outliers are sometimes real. In the second case, we may be dealing with two or more types of data related to different phenomena.
The data for this recipe is described at https://vincentarelbundock.github.io/Rdatasets/doc/robustbase/starsCYG.html (retrieved August 2015). It consists of logarithmic effective temperature and logarithmic light intensity for 47 stars in a certain star cluster. Any astronomers reading this paragraph will know the Hertzsprung-Russell diagram. In data analysis terms, the diagram is a scatter plot, but for astronomers, it is of course more than that. The Hertzsprung Russell diagram was defined around 1910 and features a diagonal line (not entirely straight) called the...