Set bubble size (z-axis)

A bubble chart is similar to a scatter chart, but the data points are replaced with bubbles. Another difference is the third dimension of the data, usually represented by the size of the bubbles. This means each data point has three different axes: x, y, and z-axis.

1. In the upper-right corner, click Edit. 
2. In the right-hand sidebar, click Data Series.
3. Click a series to expand it.
4. From the dropdown menu next to Z, select the series you want to use as the Z-axis. 

Quicker way to select a series is to click the bar, line, or symbol representing that series in the chart.