custom axisValueFormatter is always getting unset

I made a custom formatter that inherits from DefaultAxisValueFormatter so that I can print out custom labels, and I'm setting my xAxis.valueFormatter to this custom formatter. However, the get code is always re-setting it to the DefaultAxisValueFormatter because these conditions are always being met:

~~~~
get
{
if _axisValueFormatter == nil ||
(_axisValueFormatter is DefaultAxisValueFormatter &&
(_axisValueFormatter as! DefaultAxisValueFormatter).hasAutoDecimals &&
(_axisValueFormatter as! DefaultAxisValueFormatter).decimals != decimals)
{
_axisValueFormatter = DefaultAxisValueFormatter(decimals: decimals)
}

return _axisValueFormatter
}
~~~~

I got around it by not inheriting, but it seems like an issue others might run into too.