BarChart displaying more bars than inside the dataset

We just migrated from an older charts version and the following strange behaviour occurs.

We are creating the dataset like this:

        var yVals:[BarChartDataEntry] = []
        let statistic = MPStatistic.createStatistic(fromDate, untilDate: untilDate)
        var maxCount = 0
        if statistic.days.count > 0 {
            let dayWithMaxTrainingsPossible = statistic.days.reduce(statistic.days[0]) {
                ($0.trainingsCountMax > $1.trainingsCountMax) ? $0 : $1
            }
            maxCount = dayWithMaxTrainingsPossible.trainingsCountMax
        }
        for (index, day) in statistic.days.enumerated() {
            DDLogDebug("\(index) : \(day)")
            let entry = BarChartDataEntry(x: Double(index), y: (Double(day.trainingsCountDone) / Double(maxCount) * 100.0))
            yVals.append(entry)
        }
        let barChartDataSet = BarChartDataSet(values: yVals, label: nil)
        barChartDataSet.colors = [UIColor.mpGreenColor()]
        barChartDataSet.drawValuesEnabled = false
        DDLogDebug("DataSetCount stacks: \(barChartDataSet.entryCountStacks) and DataSetCount stack size: \(barChartDataSet.stackSize)")
        
        let barChartData = BarChartData(dataSet: barChartDataSet)
        
        let leftAxis: YAxis = barChartView.leftAxis
        leftAxis.labelCount = 5
        leftAxis.axisMaximum = Double(100)
        leftAxis.axisMinimum = Double(0)
        
        barChartView.xAxis.labelCount = 7
        barChartView.data = barChartData
        barChartView.notifyDataSetChanged()

And we get the following log:

2018-01-21 19:23:14:898 app[6838:224957] 0 : MPStatisticDay
2018-01-21 19:23:14:898 app[6838:224957] 1 : MPStatisticDay
2018-01-21 19:23:14:898 app[6838:224957] 2 : MPStatisticDay
2018-01-21 19:23:14:898 app[6838:224957] 3 : MPStatisticDay
2018-01-21 19:23:14:898 app[6838:224957] 4 : MPStatisticDay
2018-01-21 19:23:14:898 app[6838:224957] 5 : MPStatisticDay
2018-01-21 19:23:14:898 app[6838:224957] 6 : MPStatisticDay
2018-01-21 19:23:14:899 app[6838:224957] DataSetCount stacks: 7 and DataSetCount stack size: 1

So i assume, there are 7 entries in the data set.

But when we see the chart, there are 9 entries, starting with -1. How is this happening?

We had this before migration: xAxis.setLabelsToSkip(2) but this function is missing now.

The graph looks like this:

Any ideas on this strange thing? thx