Bubble Plot

The symbols (also called bubbles) on a bubble plot represent the following three values for each observation:

· The position of the bubble on the y-axis represents the value of one continuous variable.

· The position of the bubble on the x-axis represents the value of a second continuous variable.

· The size of the bubble—more specifically, the area of the bubble—represents the value of a third continuous variable.

In addition, you can use data labels to show the value of a fourth variable, which can be either continuous or categorical. And you can use symbols of different colors to represent the values of up to three categorical grouping variables.

Note	Because bubble size can represent only positive values, bubble plots do not show observations for which the bubble size values are less than or equal to 0. You can customize the size and other attributes of bubbles after you create the graph. Select the bubbles and choose Editor > Edit Bubbles.

Use a bubble plot to look for relationships among the variables, such as the following:

· Areas of the graph where bubbles tend to be smaller or larger.

· Areas on the graph where bubbles or groups are clustered.

· Variables that tend to increase or decrease in value together.

· Individual bubbles that differ in size from their neighbors.

Example Output

$image\gr_3Dplot.gif$

Interpretation

The bubble plot shows that Highway (Hwy) and City mileage are highly correlated. Cars with better city mileage also tend to have better highway mileage.

The red bubbles represent hybrid vehicles, which use a combination of electric power and gas. The blue bubbles represent vehicles that are powered only by gas. Not surprisingly, all 5 of the cars that have the best mileage are hybrid vehicles. Hybrid cars appear to be especially beneficial for city driving. The best hybrid gets approximately 25 more miles per gallon than does the best gas vehicle. For highway driving, the difference is less. The best hybrid gets approximately 10 more miles per gallon than does the best gas vehicle.

Most of the larger bubbles belong to SUVs (sport utility vehicles), which indicates that SUVs are the most expensive vehicles. SUVs also tend to get the fewest miles per gallon, because they weigh more than most other vehicles. The hybrid that has the worst mileage is an SUV.