As described in the article "A five dimensional scatterplot" it is only possible to display three dimensions with Google's chart API: x, y and size of the dots. In order to provide more information to the reader of the chart, tooltips are the solution at hand.

These tooltips can be realized using the HTML area tag. For any image you can provide a map, which lets you specify certain mouse sensitive areas with different shapes like rectangle, circle or polygon. So this is exactly what we need in order to install tooltips to a google chart.
In order to see what information is needed a short inventory helps.

We have:

• the height and width of the target image
• the domain values that should be mapped to the target dimension
• the maximum and the minimum of the domain values

We need:

• the mapped coordinates of the domain values

The mapping of the domain values to coordinates can be achieved easily with the following mapping function:

coordinate = (value – min / max – min) * target size

which essentially maps the domain value into a range between 0 and 1 and then multiplies it with the available target image size (width or height), such that the maximal value is mapped to the maximal available coordinate. Since Google always starts the axes with 0, the mapping function gets even easier:

coordinate = (value / max) * target size

This coordinates can be used to create the map with the area tags that will be mouse sensitive. This sensitivity is used to show tooltips with detailed information about this data point as shown below.

Click on the image to see the interactive version of the tooltips for Google's chart API with an example scatter plot.

The chart was created with the following parameters:

chs (chart size):

300x300

cht (chart type):

s (scatter plot)

chm (chart shape marker):

20 maximal size

chxt (chart axis type):

x,y (y and y axis)

chxl (chart axis label):

0:|1.4|2.5|6.4|1:|0.2|0.95|2.1 (min, middle, and max values per axis)

chtt (chart title):

Iris+Data+Sample+Scatter+Plot

chd (chart data):

s: and the encoded data using simple encoded data

More information about the parameters you find on Google's chart API site.

Side note:
Saved me some more hours of desperate nightly search:
http://www.webmasterworld.com/html/3142618.htm
To make it work in IE7 the “usemap” attribute in the javascript has to be “useMap”.