This section regroups several chart types used to study the joint distribution of 2 numerical variables. All these methods are really useful to avoid over plotting in a scatterplot. If you have too many dots, they basically counts the number of observations
within a particular area of the 2D space and represent this count by a color. If you divide the space by several squares you get a 2D histogram. If you use hexagons you get a hexbin plot. You can also calculate Density estimate and represent 2D density plots
or Contour plots. Fortunately, the ggplot2 library has awesome geoms to easily produce this kind of charts. Note that it is a good practice to show the marginal distributions of both variables.
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