A bubble plot is a scatterplot where a third dimension is added: the value of an additional variable is represented through the size of the dots. You need 3 numerical variables as input: one is represented by the X axis, one
by the Y axis, and one by the size. Do not forget to provide a legend to make possible the link between the size and the value. Note that too many bubble make the chart hard to read, so this type of representation is usually
not recommended for big amount of data. Last but not least, note that the area of the circles must be proportional to the value, not to the radius, to avoid exaggerate the variation in your data.