AxisBase Value Formatter

[jjatie]

valueFormatter is implemented as so:

    @objc open var valueFormatter: IAxisValueFormatter?
    {
        get
        {
            if _axisValueFormatter is DefaultAxisValueFormatter,
                (_axisValueFormatter as! DefaultAxisValueFormatter).hasAutoDecimals,
                (_axisValueFormatter as! DefaultAxisValueFormatter).decimals != decimals
            {
                _axisValueFormatter = DefaultAxisValueFormatter(decimals: decimals)
            }
            
            return _axisValueFormatter
        }
        set
        {
            _axisValueFormatter = newValue ?? DefaultAxisValueFormatter(decimals: decimals)
        }
    }

If we allow custom formatters, then why do we reset the formatter if it's using a modified DefaultAxisValueFormatter? Any subclasses of DefaultAxisValueFormatter would also trigger this reset. I feel we should either not allow custom formatters, or provide an implementation like this:

    @objc open var valueFormatter: IAxisValueFormatter?
    {
        get { return _axisValueFormatter }
        set { _axisValueFormatter = newValue ?? DefaultAxisValueFormatter(decimals: decimals) }
    }

[liuxuan30]

I guess because DefaultAxisValueFormatter is kind of a demo formatter, or default one. People who want to customize usually does not subclass DefaultAxisValueFormatter?
allowing custom formatters is fine, since it's just a protocol.
Anyway, what confuses me now is the getter struggling with decimals, which I don't remember why..
if changing it won't trigger any CI failures, I think it's fine to go ahead.