A bubble chart allows you to create a visual representation of a series of data that has three values associated with it, for example, a collection of data that included the price, quality rating and sales. It is similar to a scatter plot with the third value being represented by the size of a bubble whose center represents the other two data values.
Arrange your data in three columns. The first column will be the X-axis, the second will be the Y-axis, and the third column will be the bubble size. As an example take the following set of data: Column one is price (40,20,5,15); column two is quality (5, 3, 1, 2), and column three is sales (100, 75, 200, 25).
Draw an X and Y axis for the data. Create a scale on each axis that is scaled to your data with each mark being an equal distance apart. The X-axis scale for the example data set is -10 to 50 in increments of 10. The scale is larger than the data range to allow for the bubbles to fit. The Y-axis for the example is 0 to 6 in increments of 1 and again allows for bubble size.
Find where the X and Y values for each row intersect and put a dot there. This is the center for each bubble. In the example, the center for each bubble is at (40, 5), (20, 3), (5, 1) and (15, 2).
Find the ratio of the values in the third column. The radius of each bubble will be based on the ratio of the third column value compared to the other values. First, determine how large the largest bubble will be and then make the other proportional to it. For the example, the largest bubble is third because sales for the third row is 200. The ratios are (100/200, 75/200, 200/200, 25/200) = (0.5, 0.375, 1, 0.125).
Multiply the ratio by the radius of the largest bubble to get the radius of each bubble. The size of the largest bubble is at your discretion but should fit reasonably in the graph. Continuing the example, if the largest bubble had a radius of 0.5 inches, then each bubble would have a radius of (0.5 x 0.5, 0.375 x 0.5, 1 x 0.5, 0.125 x 0.5) = (0.25, 0.1875, 0.05, 0.0625).
Draw each bubble with the radius determined above, centered on the corresponding dot. The finished example can be seen in the image (bubble size is not equal to that calculated above).