Add better handling of NaN Y values to XYCharts

[thevenet.fred at free.fr]

Hi, 

At the moment, adding samples with a Y value equal to Double.NaN to an XYChart with ValueAxis results in graphs with crippling issues, already well documented:

LineChart / AreaChart Series : gap in line / area when using NaN or null as value 
https://bugs.openjdk.java.net/browse/JDK-8092326

[ValueAxis] AutoRanging: Ignore NaN and null for internal calculation of dataMaxValue and dataMinValue
https://bugs.openjdk.java.net/browse/JDK-8091757 

Both of these enhancement requests are quite old and in one instance it seems a patch was even proposed, but apparently they kept being postponed for years.

I'd be happy to contribute a PR to try and address these issues, but before I start doing anything, I'd like to know if there are any specific reasons, technical or other, for the tickets above to have been postponed like that for so long (especially for JDK-8092326 where code was proposed).

Thanks!

Frederic Thevenet