How to addline to link stacked bar plot categories

I have a stacked bar chart for two variables.

ax = count[['new_category', "Count %", "Volume%"]].set_index('new_category').T.plot.bar(stacked=True) 
plt.xticks(rotation = 360)
plt.show()

I want to connect the categories with lines, so my categories are connected with lines.

You could loop through each of the bar containers, and then through each of the bars. Connecting the bar tops should give the desired plot:

from matplotlib import pyplot as plt
import pandas as pd

count = pd.DataFrame({'new_category': ['Cat A', 'Cat B'],
                      'Count %': [20, 30],
                      'Volume%': [15, 40]})
ax = count[['new_category', "Count %", "Volume%"]].set_index('new_category').T.plot.bar(stacked=True)
for bars in ax.containers:
    prev_x = None
    prev_y = None
    for bar in bars:
        x, y = bar.get_xy()
        w = bar.get_width()
        h = bar.get_height()
        if prev_x is not None:
            ax.plot([prev_x, x], [prev_y, y + h], ls=':', color='k', lw=0.5)
        prev_y = y + h
        prev_x = x + w
ax.legend(loc='upper left', bbox_to_anchor=[1.01, 1.01])
ax.tick_params(axis='x', rotation=0)
plt.tight_layout()
plt.show()

Here is another example, using bars created by seaborn:

from matplotlib import pyplot as plt
import seaborn as sns

flights = sns.load_dataset('flights')
flights['year'] = flights['year'].astype(str)
fig, ax = plt.subplots(figsize=(12, 5))
sns.histplot(data=flights, x='year', hue='month', weights='passengers',
             multiple='stack', palette='Set3', discrete=True, shrink=0.7, ax=ax)
for bars in ax.containers:
    prev_x = None
    prev_y = None
    for bar in bars:
        x, y = bar.get_xy()
        w = bar.get_width()
        h = bar.get_height()
        if prev_x is not None:
            ax.plot([prev_x, x], [prev_y, y + h], ls=':', color='k', lw=1)
        prev_y = y + h
        prev_x = x + w
sns.move_legend(ax, loc='upper left', bbox_to_anchor=[0.02, 1], ncol=2)
sns.despine()
ax.margins(x=0.01)
ax.set_ylabel('number of passengers')
ax.set_xlabel('')
plt.tight_layout()
plt.show()