some varaible naming refactor, issue #27 - allow empty plots

...n/java/org/knowm/xchart/demo/charts/area/AreaChart01.java

...n/java/org/knowm/xchart/demo/charts/area/AreaChart02.java

...va/org/knowm/xchart/demo/charts/area/AreaLineChart03.java

...ain/java/org/knowm/xchart/demo/charts/bar/BarChart01.java

...ain/java/org/knowm/xchart/demo/charts/bar/BarChart02.java

...ain/java/org/knowm/xchart/demo/charts/bar/BarChart04.java

...ain/java/org/knowm/xchart/demo/charts/bar/BarChart05.java

...ain/java/org/knowm/xchart/demo/charts/bar/BarChart06.java

...ain/java/org/knowm/xchart/demo/charts/bar/BarChart07.java

...ain/java/org/knowm/xchart/demo/charts/bar/BarChart08.java

...ain/java/org/knowm/xchart/demo/charts/bar/BarChart09.java

...n/java/org/knowm/xchart/demo/charts/date/DateChart01.java

...n/java/org/knowm/xchart/demo/charts/date/DateChart02.java

...n/java/org/knowm/xchart/demo/charts/date/DateChart03.java

...n/java/org/knowm/xchart/demo/charts/date/DateChart04.java

...n/java/org/knowm/xchart/demo/charts/date/DateChart05.java

...n/java/org/knowm/xchart/demo/charts/date/DateChart06.java

...n/java/org/knowm/xchart/demo/charts/date/DateChart07.java

...n/java/org/knowm/xchart/demo/charts/date/DateChart08.java

...n/java/org/knowm/xchart/demo/charts/line/LineChart01.java