How to change annotations in one column of seaborn heatmap

Question

I plotted a heatmap using seaborn as shown below. I want to change the highlighted column into "Thousands K and Million M" values (shown in the table below). I tried it doing it but the column is changing into string and giving me an error when I am trying to plot those values on the heatmap.

Is there a way I can change the values of first column to the desired values on the heatmap?

Values	Desired Values
662183343.70	662.83M
155554910.90	155.55M

Code used for creating the heatmap

sns.heatmap(heatmap_df, cmap='rocket_r', annot=True, fmt='.4f', linewidths=2, 
                     cbar_kws={'label': 'Percentiles', 'orientation': 'vertical', },
                     vmin=0.95, vmax=1, xticklabels=xticks_label, ) 

Changing the column into K, M format

# convert zeros to K, M etc.
from numerize import numerize as nz

df['PropDmgAdj'] = df['PropDmgAdj'].apply(nz.numerize)

Answer 1

• sns.heatmap will throw a ValueError if trying to plot a DataFrame with strings. Therefore, create a custom DataFrame, annot, to pass to the annot= parameter.
• Format the numbers in annot with .round(4), because fmt='' must be used in the plot call.
• Tested in python 3.10, pandas 1.4.3, matplotlib 3.5.1, seaborn 0.11.2

import pandas as pd
import seaborn as sns
import numpy as np
import matplotlib.pyplot as plt

# sample data
np.random.seed(2022)
data = np.random.random(size=(11, 5))
data[:, 0] = data[:, 0] * 10_000_000

# sample dataframe
df = pd.DataFrame(data)

# create a dataframe to use for annotations
annot = df.copy()

# format the desired column; 0 here is the name of the column
annot[0] = annot[0].div(1000000).round(4).astype(str) + 'M'

# format the rest of the numbers in the dataframe
annot = annot.round(4)

# plot
fig, ax = plt.subplots(figsize=(8, 6))
sns.heatmap(data=df, annot=annot, fmt='', vmin=0, vmax=1, linewidths=2,
            cbar_kws={'label': 'Percentiles', 'orientation': 'vertical'}, ax=ax)

Data Views

data

np.array([[9.35861381e+04, 4.99057811e-01, 1.13383690e-01, 4.99740182e-02, 6.85407594e-01],
          [4.86988068e+06, 8.97657226e-01, 6.47452071e-01, 8.96963123e-01, 7.21134929e-01],
          [8.31353421e+06, 8.27568069e-01, 8.33579584e-01, 9.57044336e-01, 3.68044437e-01],
          [4.94837630e+06, 3.39509475e-01, 6.19429326e-01, 9.77529638e-01, 9.64330776e-02],
          [7.44206212e+06, 2.92499474e-01, 2.98675351e-01, 7.52473473e-01, 1.86637277e-02],
          [5.23737436e+06, 8.64435847e-01, 3.88842840e-01, 2.12191849e-01, 4.75180704e-01],
          [5.64672418e+06, 3.49429296e-01, 9.75908627e-01, 3.78200437e-02, 7.94269686e-01],
          [3.57882602e+06, 7.47963953e-01, 9.14509307e-01, 3.72662424e-01, 9.64883473e-01],
          [8.13857731e+05, 4.24509911e-02, 2.96796033e-01, 3.63703625e-01, 4.90255176e-01],
          [6.68518738e+06, 6.73414630e-01, 5.72100640e-01, 8.05922429e-02, 8.98331264e-01],
          [3.83885272e+05, 7.82194421e-01, 3.66563567e-02, 2.67183848e-01, 2.05223845e-01]])

df

               0         1         2         3         4
0   9.358614e+04  0.499058  0.113384  0.049974  0.685408
1   4.869881e+06  0.897657  0.647452  0.896963  0.721135
2   8.313534e+06  0.827568  0.833580  0.957044  0.368044
3   4.948376e+06  0.339509  0.619429  0.977530  0.096433
4   7.442062e+06  0.292499  0.298675  0.752473  0.018664
5   5.237374e+06  0.864436  0.388843  0.212192  0.475181
6   5.646724e+06  0.349429  0.975909  0.037820  0.794270
7   3.578826e+06  0.747964  0.914509  0.372662  0.964883
8   8.138577e+05  0.042451  0.296796  0.363704  0.490255
9   6.685187e+06  0.673415  0.572101  0.080592  0.898331
10  3.838853e+05  0.782194  0.036656  0.267184  0.205224

annot

          0       1       2       3       4
0   0.0936M  0.4991  0.1134  0.0500  0.6854
1   4.8699M  0.8977  0.6475  0.8970  0.7211
2   8.3135M  0.8276  0.8336  0.9570  0.3680
3   4.9484M  0.3395  0.6194  0.9775  0.0964
4   7.4421M  0.2925  0.2987  0.7525  0.0187
5   5.2374M  0.8644  0.3888  0.2122  0.4752
6   5.6467M  0.3494  0.9759  0.0378  0.7943
7   3.5788M  0.7480  0.9145  0.3727  0.9649
8   0.8139M  0.0425  0.2968  0.3637  0.4903
9   6.6852M  0.6734  0.5721  0.0806  0.8983
10  0.3839M  0.7822  0.0367  0.2672  0.2052

Answer 2

Using the lesser known texts attribute of matplotlib.Axes object:

import pandas as pd
import seaborn as sns
import matplotlib.pyplot as plt

heatmap_df = pd.DataFrame(
    [
        {"colA": 662183343.70, "colB": 0.9976, "colC": 0.9962},
        {"colA": 155567736.90, "colB": 1.0000, "colC": 1.0000},
        {"colA": 77777.70, "colB": 0.9976, "colC": 0.9962},
        {"colA": 14456.20, "colB": 0.1243, "colC": 0.5356},
    ]
)
heatmap = sns.heatmap(
    heatmap_df,
    cmap="rocket_r",
    annot=True,
    fmt=".4f",
    linewidths=2,
    cbar_kws={"label": "Percentiles", "orientation": "vertical",},
    vmin=0.95,
    vmax=1,
)

# https://stackoverflow.com/questions/37602885/adding-units-to-heatmap-annotation-in-seaborn
for t in heatmap.texts:
    current_text = t.get_text()
    
    # https://stackoverflow.com/questions/67629794/pandas-format-large-numbers
    text_transform = (
        lambda x: f"{x//1000000000}B"
        if x / 1000000000 >= 1
        else f"{x//1000000}M"
        if x / 1000000 >= 1
        else f"{int(x//1000)}K"
        if x / 10000 >= 1
        else f"{x}"
    )
    t.set_text(text_transform(float(current_text)))


plt.show()