A 2D density plot  or  2D histogram is an extension of the well known histogram. It shows the distribution of values in a data set across the range of two quantitative variables. It is really

useful to avoid over plotting in a scatterplot. If you have too many dots, the 2D density plot counts the number of observations within a particular area of the 2D space. This specific area can be

a square or a hexagon (hexbin). You can also estimate a 2D kernel density estimation and represent it with contours.

Contour plot

2D Histogram