For reference, this is the code that I currently use to create the graph initially and redraw it:
// (1) initial creation & draw
data = new google.visualization.DataTable();
chart = new google.visualization.LineChart(document.getElementById('chart'));
data.addColumn('date', 'Time');
data.addColumn('number', 'Value');
chart.draw(data, options);
// (2) new data arrived, insert & truncate, redraw
data.addRow([someTime, someFloat)])
if (data.getNumberOfRows() > threshold) {
data.removeRow(0);
}
chart.draw(data, options)