Parameters: 
 x : 1d array of dimension N
 y : 2d array (dimension MxN), or sequence of 1d arrays (each dimension 1xN)
The data is assumed to be unstacked. Each of the following
calls is legal:
stackplot(x, y) # where y is MxN
stackplot(x, y1, y2, y3, y4) # where y1, y2, y3, y4, are all 1xNm
 baseline : {'zero', 'sym', 'wiggle', 'weighted_wiggle'}
Method used to calculate the baseline:
'zero' : Constant zero baseline, i.e. a simple stacked plot.
'sym' : Symmetric around zero and is sometimes called
'ThemeRiver'.
'wiggle' : Minimizes the sum of the squared slopes.
'weighted_wiggle' : Does the same but weights to account for
size of each layer. It is also called 'Streamgraph'layout. More
details can be found at http://leebyron.com/streamgraph/.
 labels : Length N sequence of strings
Labels to assign to each data series.
 colors : Length N sequence of colors
A list or tuple of colors. These will be cycled through and used to
colour the stacked areas.
 **kwargs
All other keyword arguments are passed to Axes.fill_between() .
