• This is a new feature in 5.5.0 https://github.com/plotly/plotly.py/releases. After installing and restarting my jupyter kernel, this did not work. Required restart of complete jupyter environment
• the documented example using go https://plotly.com/python/heatmaps/#text-on-heatmap-points plus note in release notes auto_text parameter, I explored this.
• below shows use of px and also go by adding numbers as text as well

import plotly.express as px
import plotly.graph_objects as go
import inflect

p = inflect.engine()
df = px.data.medals_wide(indexed=True)
fig = px.imshow(df, text_auto=True)
fig2 = go.Figure(fig.data, fig.layout)
fig2 = fig2.update_traces(text=df.applymap(p.number_to_words).values, texttemplate="%{text}", hovertemplate=None)

fig.show()
fig2.show()

dash

• see https://dash.plotly.com/external-resources Controlling the Plotly.js Version Used by dcc.Graph
• dash 2.0.0 is packaged with a version of plotly.js that does not yet support texttemplate
• code below shows how to override

import numpy as np
import dash
import plotly.graph_objects as go
import plotly.express as px
from dash.dependencies import Input, Output, State
from jupyter_dash import JupyterDash
import inflect

# Build App
app = JupyterDash(__name__, external_scripts=["https://cdn.plot.ly/plotly-2.8.3.min.js"])
app.scripts.config.serve_locally = False
app.layout = dash.html.Div(
    [
        dash.dcc.Interval(id="run", max_intervals=1),
        dash.dcc.Graph(id="fig"),
    ]
)


def hm():
    p = inflect.engine()
    df = px.data.medals_wide(indexed=True)
    fig = px.imshow(df, text_auto=True)
    fig2 = go.Figure(fig.data, fig.layout)
    fig2 = fig2.update_traces(
        text=df.applymap(p.number_to_words).values,
        texttemplate="%{text}",
        hovertemplate=None,
    )
    return fig2.update_layout(title=f"dash: {dash.__version__}")


@app.callback(Output("fig", "figure"), Input("run", "n_intervals"))
def createFig(n):
    return hm()


# Run app and display result inline in the notebook
app.run_server(mode="inline")