Bubble charts flawed
The problem with bubble charts is that the they represent numbers through the area of the bubble. This is a problem because area increases as a square of the diameter. Linear dimensions, like diameter, are what people interpret best. Expecting people to understand Pi-r-squared is unrealistic. A bubble chart that has twice the diameter does not have twice the area (it has more).
Vertical bar charts work because the horizonal dimension is fixed. The vertical dimension varies in a linear fashion compared to the others.
In your bubble chart example, you actually provided redundant information. The x axis represented the number of servers, which you also represented with the size of the bubble. Completely unnecessary.
