ntegrate.trapezoid([1, 2, 3], dx=2)
8.0

Using a decreasing ``x`` corresponds to integrating in reverse:

>>> integrate.trapezoid([1, 2, 3], x=[8, 6, 4])
-8.0

More generally ``x`` is used to integrate along a parametric curve. We can
estimate the integral :math:`\int_0^1 x^2 = 1/3` using:

>>> x = np.linspace(0, 1, num=50)
>>> y = x**2
>>> integrate.trapezoid(y, x)
0.33340274885464394

Or estimate the area of a circle, noting we repeat the sample which closes
the curve:

>>> theta = np.linspace(0, 2 * np.pi, num=1000, endpoint=True)
>>> integrate.trapezoid(np.cos(theta), x=np.sin(theta))
3.141571941375841

``trapezoid`` can be applied along a specified axis to do multiple
computations in one call:

>>> a = np.arange(6).reshape(2, 3)
>>> a
array([[0, 1, 2],
       [3, 4, 5]])
>>> integrate.trapezoid(a, axis=0)
array([1.5, 2.5, 3.5])
>>> integrate.trapezoid(a, axis=1)
array([2.,  8.])
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