bar chart fails to display when only one entry is there in dataSeries

```
 self.chartView.graphView.dataSeries = [
            OCKDataSeries(values: [132], title: "Steps", gradientStartColor: ColorUtility.chartBottomColor(), gradientEndColor: ColorUtility.chartTopColor())
        ]
```

when we have two entries in values bar seems to work fine. it doesn't show anything when only one entry is there