ChartData ValueTextColor can not achieve a variety of colors

[AflyingApple]

What did you expect to happen?

I want to set many colors for labels.

What happened instead?

But no method is found, but the Android framework（MPAndroidChart） can be implemented.

Charts Environment

**Charts version Number:3.2.0
**Xcode version:10.0
**Swift version:4.2
**Platform(s) running Charts:iOS
**macOS version running Xcode:10.14

Demo Project

[liuxuan30]

    /// - returns: The color at the specified index that is used for drawing the values inside the chart. Uses modulus internally.
    open func valueTextColorAt(_ index: Int) -> NSUIColor
    {
        var index = index
        if index < 0
        {
            index = 0
        }
        return valueColors[index % valueColors.count]
    }