Why doesn't sns.barplot legend show all values?

Question

I have the following graph. Why does the last graph legend not contain all labels? It should contain these values:

[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 20, 21]

Code:

# select only this year
year = 2022
quarter = [1, 2, 3, 4]

# check dataframe (there's no 4 quarter in 2022 year)
def is_df_empty(df):
    return len(df) != 0


# filter by 2022 year
df_year = df4_1_grouped[(df4_1_grouped['date'].dt.year == year)]
# count number of quarters in year
number_of_plots = len(df_year['date'].dt.quarter.unique())
# subplots
figure, axs = plt.subplots(number_of_plots, 1, figsize=(10, 6*number_of_plots))
axes = axs.ravel()

# loop for iter quarters
for q, ax in zip(quarter, axes):
    # filter by quarter
    df_filtered = df_year[
        df_year['date'].dt.quarter.isin(list([q]))
    ]
    # check df.empty
    if is_df_empty(df_filtered):
        # plot
        ax = sns.barplot(
            data=df_filtered,
            x='month_dt',
            y='mentor_cnt',
            hue='sessions',
            ax=ax,
            palette="BrBG",
            linewidth=0
        )
        # add values
        for container in ax.containers:
            ax.bar_label(container, size=11)

        ax.grid(False)
        ax.tick_params(axis='both', labelsize=12)
        ax.set_xlabel('')
        ax.set_ylabel('Mentors', fontsize=14)
        ax.set_ylim(0, max(df_filtered['mentor_cnt'])*1.2)
        # add legend
        ax.legend(loc='upper right', fontsize=11)
        ax.set_title(f'Session finished per month for mentors {year} year - {q} quarter', fontsize=15)

    else:
        continue
plt.tight_layout()
plt.show()

Table:

	month_dt	sessions	mentor_cnt	date
37	2022-01-01	1	133	2022-01-01 00:00:00
38	2022-01-01	2	53	2022-01-01 00:00:00
39	2022-01-01	3	18	2022-01-01 00:00:00
40	2022-02-01	1	142	2022-02-01 00:00:00
41	2022-02-01	2	52	2022-02-01 00:00:00
42	2022-02-01	3	18	2022-02-01 00:00:00
43	2022-02-01	4	1	2022-02-01 00:00:00
44	2022-02-01	5	1	2022-02-01 00:00:00
45	2022-03-01	1	164	2022-03-01 00:00:00
46	2022-03-01	2	73	2022-03-01 00:00:00
47	2022-03-01	3	36	2022-03-01 00:00:00
48	2022-03-01	4	7	2022-03-01 00:00:00
49	2022-04-01	1	175	2022-04-01 00:00:00
50	2022-04-01	2	102	2022-04-01 00:00:00
51	2022-04-01	3	39	2022-04-01 00:00:00
52	2022-04-01	4	16	2022-04-01 00:00:00
53	2022-04-01	5	4	2022-04-01 00:00:00
54	2022-04-01	6	2	2022-04-01 00:00:00
55	2022-05-01	1	168	2022-05-01 00:00:00
56	2022-05-01	2	118	2022-05-01 00:00:00
57	2022-05-01	3	45	2022-05-01 00:00:00
58	2022-05-01	4	21	2022-05-01 00:00:00
59	2022-05-01	5	13	2022-05-01 00:00:00
60	2022-05-01	6	2	2022-05-01 00:00:00
61	2022-06-01	1	179	2022-06-01 00:00:00
62	2022-06-01	2	111	2022-06-01 00:00:00
63	2022-06-01	3	52	2022-06-01 00:00:00
64	2022-06-01	4	24	2022-06-01 00:00:00
65	2022-06-01	5	6	2022-06-01 00:00:00
66	2022-06-01	6	9	2022-06-01 00:00:00
67	2022-06-01	7	3	2022-06-01 00:00:00
68	2022-07-01	1	167	2022-07-01 00:00:00
69	2022-07-01	2	129	2022-07-01 00:00:00
70	2022-07-01	3	85	2022-07-01 00:00:00
71	2022-07-01	4	38	2022-07-01 00:00:00
72	2022-07-01	5	16	2022-07-01 00:00:00
73	2022-07-01	6	11	2022-07-01 00:00:00
74	2022-07-01	7	8	2022-07-01 00:00:00
75	2022-07-01	8	1	2022-07-01 00:00:00
76	2022-07-01	9	1	2022-07-01 00:00:00
77	2022-08-01	1	155	2022-08-01 00:00:00
78	2022-08-01	2	123	2022-08-01 00:00:00
79	2022-08-01	3	91	2022-08-01 00:00:00
80	2022-08-01	4	50	2022-08-01 00:00:00
81	2022-08-01	5	34	2022-08-01 00:00:00
82	2022-08-01	6	26	2022-08-01 00:00:00
83	2022-08-01	7	16	2022-08-01 00:00:00
84	2022-08-01	8	8	2022-08-01 00:00:00
85	2022-08-01	9	5	2022-08-01 00:00:00
86	2022-08-01	10	1	2022-08-01 00:00:00
87	2022-08-01	12	1	2022-08-01 00:00:00
88	2022-09-01	1	183	2022-09-01 00:00:00
89	2022-09-01	2	105	2022-09-01 00:00:00
90	2022-09-01	3	65	2022-09-01 00:00:00
91	2022-09-01	4	32	2022-09-01 00:00:00
92	2022-09-01	5	13	2022-09-01 00:00:00
93	2022-09-01	6	12	2022-09-01 00:00:00
94	2022-09-01	7	7	2022-09-01 00:00:00
95	2022-09-01	8	6	2022-09-01 00:00:00
96	2022-09-01	9	2	2022-09-01 00:00:00
97	2022-09-01	10	1	2022-09-01 00:00:00
98	2022-09-01	11	2	2022-09-01 00:00:00
99	2022-09-01	13	1	2022-09-01 00:00:00
100	2022-09-01	20	1	2022-09-01 00:00:00
101	2022-09-01	21	1	2022-09-01 00:00:00

Answer 1

As of seaborn 0.13.0, legend entries will sometimes be hidden by default.

There is now a legend parameter to control how much is shown:

legend=...	Behavior
"brief"	Numeric hue and size variables will be represented with a sample of evenly spaced values
"full"	Every group will get an entry in the legend
"auto" (default)	Choose between brief or full representation based on number of levels
False	No legend data is added and no legend is drawn

So use legend="full" if you want to make sure all legend entries are shown:

ax = sns.barplot(
    data=df_filtered,
    x="month_dt",
    y="mentor_cnt",
    hue="sessions",
    ax=ax,
    palette="BrBG",
    legend="full" # new in seaborn 0.13.0
)