DOC: update the DataFrame.plot.line docstring

[ifosch]

Checklist for the pandas documentation sprint (ignore this if you are doing
an unrelated PR):

• PR title is "DOC: update the docstring"
• The validation script passes: scripts/validate_docstrings.py <your-function-or-method>
• The PEP8 style check passes: git diff upstream/master -u -- "*.py" | flake8 --diff
• The html version looks good: python doc/make.py --single <your-function-or-method>
• It has been proofread on language by another sprint participant

Please include the output of the validation script below between the "```" ticks:

################################################################################
#################### Docstring (pandas.DataFrame.plot.line) ####################
################################################################################

Plot DataFrame columns as lines.

This function is useful to plot lines using DataFrame's values
as coordinates.

Parameters
----------
x : int or str, optional
    Columns to use for the horizontal axis.
    Either the location or the label of the columns to be used.
    By default, it will use the `DataFrame` indices.
y : int, str, or list of them, optional
    The values to be plotted.
    Either the location or the label of the columns to be used.
    By default, it will use the remaining DataFrame numeric columns.
kwds : optional
    Keyword arguments to pass on to :py:meth:pandas.DataFrame.plot.

Returns
-------
axes : :class:matplotlib.AxesSubplot or :class:numpy.ndarray of
       them.

See Also
--------
matplotlib.pyplot.plot : Plot y versus x as lines and/or markers.

Examples
--------

.. plot::
    :context: close-figs

    The following example shows the populations for some animals
    over the years.

    >>> df = pd.DataFrame({
    ...    'pig': [20, 18, 489, 675, 1776],
    ...    'horse': [4, 25, 281, 600, 1900]
    ...    }, index=[1990, 1997, 2003, 2009, 2014])
    >>> lines = df.plot.line()

.. plot::
    :context: close-figs

    The following example shows the relationship between both
    populations.

    >>> lines = df.plot.line(x='pig', y='horse')

################################################################################
################################## Validation ##################################
################################################################################

Docstring for "pandas.DataFrame.plot.line" correct. :)

If the validation script still gives errors, but you think there is a good reason
to deviate in this case (and there are certainly such cases), please state this
explicitly.

[TomAugspurger]

I think sphinx complains if you do you have something right after a `. Just do DataFrame without the backticks.

[TomAugspurger]

Can remove this line since we discuess that in the parameters section.

[TomAugspurger]

I think x has to be an int or str. Lists are not allowed for x, just y.

[ifosch]

We tested on Jupyter and these seem to be allowed. However, the result makes not much sense. I'm changing this, anyway.

[TomAugspurger]

Remove "it". Just have "Column to use for the horizontal axis." and then the rest of your description.

[TomAugspurger]

I thin we write this as int, str, or list of them, optional.

[TomAugspurger]

"The values to be plotted."

[TomAugspurger]

No backticks. Specify that it's the remaining numeric columns.

[TomAugspurger]

:meth:np.array -> ndarray. I don't care either way about the sphinx markup so either

:class:numpy.ndarray

or

numpy.ndarray

[TomAugspurger]

Specify x='pig', y='horse'

[ifosch]

Fixes are up now

[codecov]

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[TomAugspurger]

Pushed a couple small updates.

Will wait to see if anyone else has thoughts, otherwise ready to merge.

[ifosch]

Thanks @TomAugspurger !

[TomAugspurger]

Thank you!