fixed axis title visibility bug, other misc cleanup

...n/java/com/xeiam/xchart/demo/charts/area/AreaChart01.java

...ain/java/com/xeiam/xchart/demo/charts/bar/BarChart04.java

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

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

...c/main/java/com/xeiam/xchart/internal/chartpart/Axis.java

...m/xeiam/xchart/internal/chartpart/AxisTickCalculator.java

...n/java/com/xeiam/xchart/internal/chartpart/AxisTitle.java

...c/main/java/com/xeiam/xchart/internal/chartpart/Plot.java

.../xeiam/xchart/internal/chartpart/PlotContentBarChart.java

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

...in/java/com/xeiam/xchart/internal/style/GGPlot2Theme.java

...ain/java/com/xeiam/xchart/internal/style/XChartTheme.java

xchart/src/test/java/com/xeiam/xchart/chart/Example2.java