Bubble charts are very interesting as it provides a depth to the normal 2 dimensional values.
The depth is percieved by the size of the bubble. You can create a bubble chart by providing a third column that represents the radius of the bubble. This is known as the Z axis.
select film_id x
,count(inventory_id) y1 ,count(store_id)/10 z1
, film_id y2,count(inventory_id)/5 z2
group by film_id
In the above SQL, the X-axis is common for all series of bubbles (the red bubbles represents 1 type of series and the blue bubbles represent second type of series).
The columns following X axis are the Y axis which indicates the position of bubble on X-Y and the third column represents the Z- Axis which is the radius of the bubble. The columns Y1, Z1 represents 1 series and columns Y2, Z2 represent second series