Negative Bar animation is gone if axis.inverted=true

Negative Bar animation is gone if `axis.inverted=true`

The issue is found while reviewing #3650
## What did you expect to happen?

negative bar animation should be fine when inverted is enabled

## Charts Environment

**Charts version/Branch/Commit Number:**
**Xcode version:** 10
**Swift version:** 4.2
**Platform(s) running Charts:** iOS
**macOS version running Xcode:** 10.14

## Demo Project
Just use below data set:
```swift
func setDataCount(_ count: Int, range: Double) {
let yVals = [
BarChartDataEntry(x: 0, y: -10),
BarChartDataEntry(x: 1, y: 20),
BarChartDataEntry(x: 2, y: -30),
BarChartDataEntry(x: 3, y: 15)
]

var set1: BarChartDataSet! = nil
```

and add a breakpoint or code
```swift
chartView.leftAxis.inverted = true // false
```

You will see the animation for pos. bars is fine, but the animation for neg. bars is gone if inverted = true. **animation for neg. bars is fine when inverted = false**

Start with the demo data in setDataCount and the chartView.leftAxis.inverted setting, then trace the bar animation behavior for negative and positive entries. Compare inverted=true with inverted=false and verify that negative bars animate when the axis is inverted.

Written by the indexing model from the issue text.