Hi Sascha,

First and foremost sincerely regret delayed reply.

included chart library from https://www.gstatic.com/charts/loader.js instead of https://www.google.com/jsapi (https://www.gstatic.com/charts/loader.js instead of https://www.google.com/jsapi). So the includeScript will now be:

<apex:includeScript id="a" value="https://www.gstatic.com/charts/loader.js" />

Then, at the end of the code where you are doing:

var chart = new google.charts.Bar(document.getElementById('chart_div'));
    chart.draw(data, google.charts.Bar.convertOptions(options));

Replace the above code as below:

var chart = new google.visualization.ColumnChart(document.getElementById('chart_div'));
          chart.draw(data, options);

One more point is, you have to add the annotation column for each of the row you want to show the annotation. So, here is the complete code.

<apex:page doctype="html-5.0" controller="PieChartController">
<apex:includeScript id="a" value="https://www.gstatic.com/charts/loader.js" />

<apex:pageblock title="Samplepage">

<div id="chart_div"></div>

<script type="text/javascript">

google.charts.load('current', {packages: ['corechart', 'bar']});

google.charts.setOnLoadCallback(drawStuff);

function drawStuff() {
    var data = new google.visualization.DataTable();
    data.addColumn('string', 'Team');
    data.addColumn('number', 'Level 1');
    data.addColumn({type: 'number', role: 'annotation'});
    data.addColumn('number', 'Level 2');
    data.addColumn({type: 'number', role: 'annotation'});
    data.addColumn('number', 'Level 3');
    data.addColumn({type: 'number', role: 'annotation'});
    data.addColumn('number', 'Level 4');
    data.addColumn({type: 'number', role: 'annotation'});
    data.addColumn('number', 'YTD');    
    data.addColumn({type: 'number', role: 'annotation'});

    data.addRows([
        ['Team1',  30, 55, 40, 66, 45, 77, 50, 88, 10, 99],
        ['Team2',  34, 11, 45, 22, 46, 33, 58, 44, 69, 97]
    ]);

    var options = {
        isStacked: true,
        width: 850,
        height: 800,
        backgroundColor: '#F8F8F8',

        chartArea: { backgroundColor: '#F8F8F8' },

        chart: {
            subtitle: 'SampleSubTitle'
        },        
        vAxis: {
            format: 'decimal',
            viewWindow: {
                min: 0,
                max: 100
            }
        },
        series: {
            4: { targetAxisIndex: 1 }, 
            5: { targetAxisIndex: 1 }
        }
    };

    var chart = new google.visualization.ColumnChart(document.getElementById('chart_div'));
      chart.draw(data, options);
};   
</script>
</apex:pageblock>
</apex:page>

Kindly mark this post as solved if the information help's so that it gets removed from the unanswered queue which results in helping others who are really in need of it.

Best Regards,
Nagendra.P