Google Visualization data.join method leads to tooltips being removed
<p>I'm merging two datasets using the <code>google.visualization.data.join</code> method. This works well apart from one of the data columns seemingly being deleted in the process.</p>

<p>More specifically, my datasets are built like this:</p>

<pre><code>var data1 = new google.visualization.DataTable();

data1.addColumn('date', 'Date');
data1.addColumn('number', 'Amount');
data1.addColumn({type: 'string', role: 'tooltip'});

// Similar code for data2
</code></pre>

<p>I then proceed to join the two datasets like this:</p>

<pre><code>var joinedData = google.visualization.data.join(data1, data2, 'full', [[0, 0]], [1], [1]);
</code></pre>

<p>And then initialize the chart like this:</p>

<pre><code>// Create and draw the visualization.
var chart = new google.visualization.ScatterChart(
document.getElementById('chart-div'));
chart.draw(joinedData, {
    interpolateNulls: true,
    title: 'Test',
    width: 600, height: 400,
    vAxis: {title: "cr", titleTextStyle: {color: "green"}},
    hAxis: {title: "time", titleTextStyle: {color: "green"}},
    lineWidth: 1}
);
</code></pre>

<p>For some reason the tooltips are no longer visible when I hoved the mouse over the data rows. They were visible when I just presented one set alone.</p>