**2. General principles of animating bubble charts**

In Excel® and other spreadsheet programs, graphs added to a worksheet can be updated automatically and almost instantaneously when the content of the worksheet is altered. This enables animations driven by a macro that achieves step-by-step changes in the content of a range of worksheet cells. The speed of an animation can be controlled by making calls to a special function that puts the macro to sleep and wakes it up after a specified amount of time.

Because visual inspection is particularly suitable for detecting motion against a static background, we developed animations in which all data are used to construct a static background, and different subsets of data are sequentially highlighted. In a 2D bubble chart, this type of displays can be constructed by using open markers for the static background and filled markers for the highlighted data. This is illustrated in Figure 1, which shows how the interdependence between reported pH and alkalinity levels in the Baltic Proper has changed over time. In particular, it can be noted that the reported interdependence changed dramatically from 1989–1993 to 1994–1998, most probably due to changes in laboratory practices.
