BUG: Make xticks from _quarterly_finder() line up better
When I plot a series of quarterly data that spans more than 11 years, my vertical grid lines get placed a year before where they should, i.e., one year before a year that is divisible by the default annual spacing. Changing one line in the _quarterly_finder() function, from +1 to +1970, fixes this for me. Can anyone please confirm the issue and that this fixes it?
Hey, I'm not too familiar with the project's test infrastructure, so I apologize if this isn't too convenient, but here's a standalone test that works for me.

```
import pandas as pd
import numpy as np
from pandas import Period
from pandas.plotting._matplotlib import converter


def test_quarterly_finder(data):
    vmin = data[0]
    vmax = data[-1]
    (vmin, vmax) = (int(vmin), int(vmax))
    if vmin > vmax:
        (vmin, vmax) = (vmax, vmin)
    span = vmax - vmin + 1
    if span < 45:  # the quarterly finder is only invoke if the span is >= 45
        return
    nyears = span / 4
    (min_anndef, maj_anndef) = converter._get_default_annual_spacing(nyears)
    result = converter._quarterly_finder(data[0], data[-1], 'Q')
    quarters = pd.PeriodIndex(pd.arrays.PeriodArray(
        np.array([x[0] for x in result]), freq='Q'))
    majors = np.array([x[1] for x in result])
    minors = np.array([x[2] for x in result])
    major_quarters = quarters[majors]
    minor_quarters = quarters[minors]
    check_major_years = major_quarters.year % maj_anndef == 0
    check_minor_years = minor_quarters.year % min_anndef == 0
    check_major_quarters = major_quarters.quarter == 1
    check_minor_quarters = minor_quarters.quarter == 1
    assert (np.all(check_major_years) and np.all(check_minor_years)
            and np.all(check_major_quarters) and np.all(check_minor_quarters))


start_date = pd.to_datetime('1947-01-01')
span15 = pd.date_range(start_date, periods=15*4,
                       freq='Q').to_period(freq='Q').asi8
span30 = pd.date_range(start_date, periods=30*4,
                       freq='Q').to_period(freq='Q').asi8
span80 = pd.date_range(start_date, periods=80*4,
                       freq='Q').to_period(freq='Q').asi8
span150 = pd.date_range(start_date, periods=150*4,
                        freq='Q').to_period(freq='Q').asi8

test_quarterly_finder(span15)
test_quarterly_finder(span30)
test_quarterly_finder(span80)
test_quarterly_finder(span150)
``` Please review https://pandas.pydata.org/pandas-docs/stable/development/contributing_codebase.html#test-driven-development-code-writing for adding unit tests. We cannot accept bug fixes without a unit test Hello @elidourado! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:





There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2022-07-15 17:28:32 UTC Let me know what else I can do to get this committed! Done. I had to make a new visualization section in the 1.5.0 what's new file. There is an existing plotting section. Would it be better to consolidate and move the note there?  Thanks for sticking with this @elidourado 