Annotate just specific windows of imshow heatmaps with marks (e.g. “x”)

Is it possible to annotate imshow heatmap the way that if the value from pandas Dataframe is e.g. less than 3, then make mark “x” in that specific heatmap window?

Lets assume I have similar data to this example:

d = {'col1': [1,2,1,5,3], 'col2': [3,4,1,5,2],'col3': [3,4,3,1,2],'col4': [3,4,1,2,5]}
df = pd.DataFrame(data=d)

fig = plt.figure(figsize=(6,4))
plot = plt.imshow(df)
ax = plot.axes
ax.invert_yaxis()
ax.set_xticklabels(df.columns)

I saw that we can annotate all heatmap windows with corresponding values, however I can’t figure out how to annotate just the windows below my given limit and not with the corresponding value but e.g. with mark “x”. Can I ask for suggestions if it is possible with matplotlib heatmaps, please? Thank you.

Answer

• Iterate through the values, and use an if-condition to control adding the text.
• Modified from how to annotate heatmap with text in matplotlib

d = {'col1': [1,2,1,5,3], 'col2': [3,4,1,5,2],'col3': [3,4,3,1,2],'col4': [3,4,1,2,5]}
df = pd.DataFrame(data=d)

fig = plt.figure(figsize=(7, 7))
plot = plt.imshow(df)
ax = plot.axes
ax.invert_yaxis()
ax.set_xticks(range(len(df.columns)), df.columns)

for y in range(df.shape[0]):
    for x in range(df.shape[1]):
        val = df.iloc[y, x]
        if val >= 3:
            val = 'x'
        plt.text(x, y, val, fontsize=14,
                 horizontalalignment='center',
                 verticalalignment='center')