fix(stackedplot): address an issue where stack plot doesn't work

[dakshpokar]

Bug Report

Description

We have two stacked plot examples in: example/stacked/matplotlib/example_mpl_stacked.py and example/stacked/seaborn/example_sns_stacked.py. Both of which are failing. @SaaiVenkat has mentioned that this has happened with the new API design.

Steps to Reproduce

1. Run the example/stacked/seaborn/example_sns_stacked.py or example/stacked/matplotlib/example_mpl_stacked.py
2. Check the output, examples fail to run.

Error/Stack-Trace Information

Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
  File "<stdin>", line 3, in main
  File "/Users/dakshpokar/RA/py_maidr/maidr/api.py", line 29, in stacked
    return FigureManager.create_maidr(ax, PlotType.STACKED)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/dakshpokar/RA/py_maidr/maidr/core/figure_manager.py", line 53, in create_maidr
    raise ValueError("No plot found.")
ValueError: No plot found.

Additional Information

Cannot add an example file for the stacked plot in #64 because of this issue.