Variable names should generally be lowercase

In many places we have uppercase variables, e.g.

X = np.linspace(0, 2 * np.pi, 100)
Y = np.sin(X)

and likewise then in the plot functions.

This is against PEP-8 naming conventions and IMHO sets a bad example. At least 1D arrays should be lowercase. One may debate on 2D arrays, which have a history of using uppercase letters in the Matplotlib docs.