You are not required to draw the chart on page load, you just have to make sure that it doesn't start until after the API has finished loading. You can do this with AJAX requests as long as you are loading something other than the API with the AJAX. Here's a rough workup you could use (uses jQuery's AJAX function to fetch a json representation of a DataTable from a php script and draw a chart):
google.load('visualization', '1', {packages: ['corechart']});
google.setOnLoadCallback(init);
function init () {
$.ajax({
url: 'path/to/data.php',
type: 'json',
success: function (json) {
// do something with json
// the draw the chart here, maybe:
var data = new google.visualization.DataTable(json);
var chart = new google.visualization.LineChart(document.getElementById('myChart'));
chart.draw(data, {
title: 'My Chart'
});
}
});