Use a plot default method instead of overriding the init method in examples

[rahulporuri]

The chaco examples create the displayed plot object in atleast two different ways - they either override the __init__ method of the underlying traits class to create and store the plot object or they define a plot trait and a _plot_default method which creates the plot object.

I think the latter way is preferred over the former - we don't want to unnecessarily override the __init__ method.

chaco/examples/user_guide/h_plot_container.py

Lines 20 to 36 in e5a8594

	def __init__(self):
	# Create the data and the PlotData object
	x = linspace(-14, 14, 100)
	y = sin(x) * x ** 3
	plotdata = ArrayPlotData(x=x, y=y)
	# Create a scatter plot
	scatter_plot = Plot(plotdata)
	scatter_plot.plot(("x", "y"), type="scatter", color="blue")
	# Create a line plot
	line_plot = Plot(plotdata)
	line_plot.plot(("x", "y"), type="line", color="blue")
	# Create a horizontal container and put the two plots inside it
	container = HPlotContainer(line_plot, scatter_plot, spacing=100)
	self.plot = container

chaco/examples/user_guide/power_function_example.py

Lines 26 to 48 in e5a8594

	def _plot_default(self):
	y = self.x ** self.power
	plot_data = ArrayPlotData(x=self.x, y=y)
	plot = Plot(plot_data)
	plot.plot(("x", "y"), "line", name="power function", color="auto")
	# configure the plot
	plot.padding_top = 25
	plot.border_visible = False
	plot.index_grid.visible = False
	plot.value_grid.visible = False
	plot.title = "Power Function n={}".format(self.power)
	plot.title_position = "right"
	plot.title_angle = -90
	plot.legend_alignment = "ul"
	plot.legend.border_visible = False
	plot.legend.bgcolor = (0.9, 0.9, 0.9, 0.5)
	plot.legend.visible = True
	plot.index_axis.title = "y"
	plot.value_axis.title = "x"
	return plot