Refactoring plot, add element.
Merge branch 'master' of
https://github.com/azuki-framework/azuki-chart.git

Conflicts:
	src/main/java/org/azkfw/chart/charts/bar/BarChartPlot.java
	src/main/java/org/azkfw/chart/charts/line/LineChartPlot.java
	src/main/java/org/azkfw/chart/charts/pie/PieChartPlot.java
	src/main/java/org/azkfw/chart/charts/polar/PolarChartPlot.java
	src/main/java/org/azkfw/chart/charts/polararea/PolarAreaChartPlot.java
	src/main/java/org/azkfw/chart/charts/radar/RadarChartPlot.java
	src/main/java/org/azkfw/chart/charts/scatter/ScatterChartPlot.java
	src/main/java/org/azkfw/chart/charts/spectrum/SpectrumChartPlot.java
	src/main/java/org/azkfw/chart/core/plot/AbstractChartPlot.java