More Customizable + Heat Map

New features in V0.1.5

[1] Line Charts, Bar Charts and Scatter Plots can deal with NA values now.

[2] Added font.size.legend argument for all charts with legend bar applicable, to help users better customize the legend bar.

[3] Added arguments font.size.axis.x, font.size.axis.y, rotate.axis.x, and rotate.axis.y for line charts, bar charts, and scatter plots. These arguments can help users better customize the charts.

[4] Added Heat Map function.