Allow null Y-Axis values for issue #35: https://github.com/timmolter/XChart/issues/35

...-demo/src/main/java/com/xeiam/xchart/demo/XChartDemo.java

...n/java/com/xeiam/xchart/demo/charts/area/AreaChart02.java

...ain/java/com/xeiam/xchart/demo/charts/bar/BarChart02.java

xchart/src/main/java/com/xeiam/xchart/Series.java

...xeiam/xchart/internal/chartpart/PlotContentLineChart.java