Joining columns of two queries

Question

You declare data0 and data1 in global scope and then create local scope variants of each in your handleQueryResponse functions, so while data0 exists in handleQueryResponse1, it is null. Even if you used the global-scope variables, this wouldn't solve your problem, as it is entirely possible that query1 would return before query0, and thus data0 would still be null.

Try this approach instead:

function drawChart() {
    // create an array containing one false for each query
    var ready = [false, false];
    // create an array to hold the DataTables
    var dataArray = [];
    function handleQueryResponse(response, index) {
        if (response.isError()) {
            alert('Error in query ' + index + ': ' + response.getMessage() + ' ' + response.getDetailedMessage());
            return;
        }
        ready[index] = true;
        dataArray[index] = response.getDataTable();
        var allReady = true;
        // check if all the queries have returned
        for (var i = 0; i < ready.length; i++) {
            if (!ready[i]) {
                allReady = false;
                break;
            }
        }
        // if all the queries have returned, draw the charts and tables
        if (allReady) {
            var data = new google.visualization.data.join(data[1], data[0], 'full', [0], [1], [1]);
            var options = {
                title: 'Electrolyser Data',
                legend: {position: 'bottom'},
                hAxis: {
                    title: 'Date',
                    titleTextStyle: {
                        color: 'red'
                    },
                    format: 'MMM d',
                    gridlines: {
                        count: 31
                    }
                }
            };
            var tables = [];
            tables[0] = new google.visualization.Table(document.getElementById('table_div0'));
            tables[0].draw(dataArray[0]);
            tables[1] = new google.visualization.Table(document.getElementById('table_div1'));
            tables[1].draw(dataArray[1]);
            tables[2] = new google.visualization.Table(document.getElementById('table_div2'));
            tables[2].draw(data);

            var charts = [];
            charts[0] = new google.visualization.ColumnChart(document.getElementById('visualization0'));
            charts[0].draw(dataArray[0], options);
            charts[1] = new google.visualization.ColumnChart(document.getElementById('visualization1'));
            charts[1].draw(dataArray[1], options);
        }
    }

    var query0 = new google.visualization.Query('http://docs.google.com/spreadsheet/ccc?key=0AoTfmfAJUVoSdGRIVklheHdIS1ZCaHQ1MllvM19hUWc&sheet=MonthlyAvg-A');
    var query1 = new google.visualization.Query('http://docs.google.com/spreadsheet/ccc?key=0AoTfmfAJUVoSdDhKOWRGRDNyeks1aF9jSFpHcmFfblE&sheet=MonthlyAvg-B');
    query0.setQuery('select D,E limit 7');
    query0.send(function (response) {
        handleQueryResponse(response, 0);
    });
    query1.setQuery('SELECT D,E limit 7');
    query1.send(function (response) {
        handleQueryResponse(response, 1);
    });
}
google.load('visualization', '1', {packages:['corechart'], callback: drawChart});