Change label styling.
Added expand_threshold argument for pie_plot/PiePlot.__init__. Expands a slice to a minimum value specified. Default is None because the end developer needs to tinker with this to get the right value for the graph size they are producing. On an 800x600 px graph with a radius of 4, 0.13 is about right.
Change text contents on Pie/Donut graphs to have percentage.
Add expand_threshold argument for DonutPlot.