BUG: Line plots aligned from start #57594
#57594 BUG: df.plot makes a shift to the right if frequency multiplies to n > 1

The cause of this bug is that by default all elements are generated to align to the end of
a period instead of at the start and the period technically does not end at the start of
the last period but rather just before the period subsequent to the last period. To fix this
problem we therefore passed a parameter stating that the elements should be aligned to
the start of the period instead of the end. Parameter "how=S" was added in pandas/plotting/_matplotlib/timeseries.py::maybe_convert_index(), 
on the function call data.index.asfreq(freq=freq_str, how = "S").  

Additional tests were added to test whether input data match data in plot and one test was merged into one the added test to reduce code duplication. 
Thank you for working on this @NoelMT. Do you mind adding plots to this PR (as a comment or in the description) that show how things are rendered before and after your changes please? That would give context to reviewers and it'll make reviewing much easier. Thanks!  Thanks @NoelMT for working on this. Seems like it will fix the problem with visualisation of `DataFrame` if `PeriodIndex` as index provided.
e.g. for the first scenario:
```
from pandas import *
import numpy as np

idx = period_range("01/01/2000", freq='7h', periods=4)
df = DataFrame(
    np.array([0, 1, 0, 1]),
    index=idx,
    columns=["A"],
)
df.plot()
print(df)
```

plot before:

<img width="552" alt="res_7h" src="https://github.com/pandas-dev/pandas/assets/91160475/47bccd86-a957-42e1-b021-de15e63cf85e">

plot after:

<img width="620" alt="dfplot_7h_correct" src="https://github.com/pandas-dev/pandas/assets/91160475/235ed96a-8b94-4d50-9cb4-181a387e9242">
 @NoelMT could you please add a note to `doc/source/whatsnew/v3.0.0.rst` in the section
https://github.com/pandas-dev/pandas/blob/dc19148bf7197a928a129b1d1679b1445a7ea7c7/doc/source/whatsnew/v3.0.0.rst?plain=1#L356
 This pull request is stale because it has been open for thirty days with no activity. Please [update](https://pandas.pydata.org/pandas-docs/stable/development/contributing.html#updating-your-pull-request) and respond to this comment if you're still interested in working on this. Thanks for the pull request, but it appears to have gone stale. If interested in continuing, please merge in the main branch, address any review comments and/or failing tests, and we can reopen.