It's possible, when you click on a feature you'll receive the data for the feature, you only need to collect the data returned multiple clicks(e.g. in an array ) and build the query based on these data.
For the linked example the important row is the "County"-row, it will be used to filter the results. To get results for multiple counties us a IN()-condition.
the modified drawVisualization-function may look like this:
//counties may either be
//a string(a single county)
//or an array(1 or more counties)
function drawVisualization(counties) {
counties=(typeof counties==='string')?[counties]:counties;
google.visualization.drawChart({
"containerId": "visualization",
"dataSourceUrl": "http://www.google.com/fusiontables/gvizdata?tq=",
"query": "SELECT County,'2010' FROM 1761814 WHERE County IN('"+counties.join("','")+"') and Sector='Total'",
"chartType": "ColumnChart",
"options": {
"title": 'comparision',
"height": 400,
"width": 400
}
});
}
To be able to choose between single/multi-selection there may be many ways. You may e.g. use the CTRL-key to enable the multi-selection. Set a flag when the key is pressed, when the flag is set push the new County to the array, when not create a new array with the given County