Yes, you can have "holes" in your data where a given series has no values assigned; ex, if you have two series, your DataTable can look like this:
var data = new google.visualization.DataTable();
data.addColumn('number', 'X');
data.addColumn('number', 'Y1');
data.addColumn('number', 'Y2');
data.addRows([
[0, 1, 5],
[1, 4, null],
[2, 3, 9],
[3, null, 4],
[4, 8, 2]
]);
Y2 would have no data for X=1 and Y1 would have no data for X=4.