Add annotation to specific cells in heatmap

Question

I am plotting a seaborn heatmap and would like to annotate only the specific cells with custom text.

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

data = StringIO(u'''75,83,41,47,19
                    51,24,100,0,58
                    12,94,63,91,7
                    34,13,86,41,77''')

labels = StringIO(u'''7,8,4,,1
                    5,2,,2,8
                    1,,6,,7
                    3,1,,4,7''')

data = pd.read_csv(data, header=None)
data = data.apply(pd.to_numeric)

labels = pd.read_csv(labels, header=None)
#labels = np.ma.masked_invalid(labels)

fig, ax = plt.subplots()
sns.heatmap(data, annot=labels, ax=ax, vmin=0, vmax=100)
plt.show()

The above code generates the following heatmap:

and the commented line generates the following heatmap:

I would like to show only the non-nan (or non-zero) text on the cells. How can that be achieved?

Answer 1

Use a string array for annot instead of a masked array:

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

data = StringIO(u'''75,83,41,47,19
                    51,24,100,0,58
                    12,94,63,91,7
                    34,13,86,41,77''')

labels = StringIO(u'''7,8,4,,1
                    5,2,,2,8
                    1,,6,,7
                    3,1,,4,7''')

data = pd.read_csv(data, header=None)
data = data.apply(pd.to_numeric)

labels = pd.read_csv(labels, header=None)
#labels = np.ma.masked_invalid(labels)

# Convert everything to strings:
annotations = labels.astype(str)
annotations[np.isnan(labels)] = ""

fig, ax = plt.subplots()
sns.heatmap(data, annot=annotations, fmt="s", ax=ax, vmin=0, vmax=100)
plt.show()