Think Stats CDF, mayplot

The Cdf class can be initialised with xs, and ys. It's different from initialisd by Items which ys is actually probability, and will be added up during MakeCdfFromItems().

Another interesting point is the Render() function. an exactly same y value was deliberately used twice with successive x value.The purpose is to draw a fine zigzagged line instead of a smooth continuous line. 

myplot has a bug that options can't be set to pyplot, e.g. yscale='log'. Should call myplot.Config(options) as an alternative.