How do you set percentage in Google Visualization Chart API?

chart.draw(data, {vAxis: {format:'#%'} } );

To get comma for thousands, use {format:'#,###%'}.

See http://code.google.com/apis/chart/interactive/docs/gallery/linechart.html

var options = {
        title: 'Chart Title',
        vAxis: {
            minValue: 0,
            maxValue: 100,
            format: '#\'%\''
        } 
    };

Try escaping the % sign. I've used this to just append the % sign in the y axis.