Gauge shows wrong threshold color if there is more than one data value for the series

[GDFaber]

• C3 version: 0.7.13
• D3 version: 5.15.0
• Browser: Chrome 80.0.3987.87
• OS: Windows 10

Problem description
If a gauge chart gets more than one data row/column, the values are summed up for each series, which works nicely as far as I can tell.

However there seems to be a problem finding the right level color after summing up the values: The arcs as well as the legend are colored according to the first data value instead of the sum (the tooltips show the correct color).

How to reproduce
Go to https://c3js.org/samples/chart_gauge.html and change the example code like this:

var chart = c3.generate({
    data: {
        columns: [
            ['data1', 20, 20],
            ['data2', 40, 40]
        ],
        type: 'gauge',
    },
    color: {
        pattern: ['#FF0000', '#F97600', '#F6C600', '#60B044'],
        threshold: {
            values: [30, 60, 90, 100]
        }
    }
});

[GDFaber]

I might have fixed it, but need to do more testing first to be sure it doesn't break anything else instead.