How to add mouseover tooltip in google charts sankey diagram - google-visualization

Question

and have been using Google Charts to visualize my data. I have tried to search for the answer to this question but couldn't not find anyone having the same problem as mine, or maybe my problem is really basic. Appreciate if someone can give me a hand.
I built a sankey diagram by following the steps listed in Google Charts - Sankey Diagram.
Here is my chart:
http://kuangkeng.github.io/keng-data-journalism/procurement%20project/sankey/index.html
However I would like to add tooltip to each line/path/link so when users mouseover, they can see the value and other details of the line.
According to Google Charts (the link above), I can fire a mouseover event using 'onmouseover' and 'onmouseout', but Google Charts does not show how to do it for Sankey diagram.
I then came over an example of using 'onmouseover' and 'onmouseout' in a Google Bar Chart:
https://developers.google.com/chart/interactive/docs/examples#mouseovertooltip
So I copied the code, modified and pasted into my code.
google.visualization.events.addListener(chart, 'onmouseover', barMouseOver);
google.visualization.events.addListener(chart, 'onmouseout', barMouseOut);
function barMouseOver(e) {
chart.setSelection([e]);
}
function barMouseOut(e) {
chart.setSelection([{'row': null, 'column': null}]);
}
When I mouseover, I got the error message "undefined is not a function". You can see the error message appears at the top of the chart when you mouseover the lines.
I tried to use another alternative showed by Google Charts documentation by adding a another column to my data and set its role as 'tooltip' but it didn't work for my chart because Google Sankey Diagram can only accept 3 columns.
Appreciate if someone can have a look or refer me to any solution available. Thanks.