Merge branch 'master' into develop

...emo/charts/plot/lines/Example0WithoutHorizontalLines.java

...plot/lines/Example0WithoutVerticalAndHorizontalLines.java

.../demo/charts/plot/lines/Example0WithoutVerticalLines.java

xchart/src/main/java/org/knowm/xchart/StyleManager.java

...java/org/knowm/xchart/internal/chartpart/PlotSurface.java

...in/java/org/knowm/xchart/internal/style/GGPlot2Theme.java

...ain/java/org/knowm/xchart/internal/style/MatlabTheme.java

.../src/main/java/org/knowm/xchart/internal/style/Theme.java

...ain/java/org/knowm/xchart/internal/style/XChartTheme.java

xchart/src/test/java/org/knowm/xchart/StyleManagerTest.java