One possibility is to use two dummy columns with values -2000 and 2000 to push the first useful values to 0:
var data = google.visualization.arrayToDataTable([
['Month', 'dummy1', 'dummy2', 'Bolivia', 'Ecuador', 'Madagascar', 'Papua New Guinea', 'Rwanda'],
['2004/05', -2000, 2000, 165, 938, 522, 998, 450],
['2005/06', -2000, 2000, 135, 1120, 599, 1268, 288],
['2006/07', -2000, 2000, 157, 1167, 587, 807, 397],
['2007/08', -2000, 2000, 139, 1110, 615, 968, 215],
['2008/09', -2000, 2000, 136, 691, 629, 1026, 366]
]);
and add option which removes dummy series from legend:
series: {
0: {
visibleInLegend: false
},
1: {
visibleInLegend: false
}
}
See example at jsfiddle.