How do I pass additional data to the tooltip in a sankey diagram (vue-d2b.js)?

Question

I am using vue-d2b to display a sankey chart and I'm trying to display some extra info per node and per link through the tooltip. I am unable to figure out how to do this. Here is what I've tried:

 chartConfig(chart) {
                chart.tooltip().html(function(d) {
                    console.log('hello');
                    return (d.key);
                });

                chart
                    .sankey()
                    .sankey()
                    .nodePadding(100)

                chart
                    .sankey()
                    .nodeDraggableY(true)
  }

But it says that chart.tooltip() is not a function. How do you pass some custom data to the tooltip here?. Any help with this would be great.