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Default bar width on date axes should be smarter

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
38/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Stale
Tech stack
python
Domain
data-visualization

Research direction

Start with the minimal Python example in the issue and compare the behavior with plotly.js issue #6199. Done means a single date point renders a visible bar with a sensible default width while multi-point bars retain their current behavior.

Written by the indexing model from the issue text.

Description

feature P3

This is likely a similar issue to https://github.com/plotly/plotly.js/issues/6199

Plotly version: 6.2.0

Minimal Example 
import plotly.graph_objects as go
import pandas as pd
import datetime

data = [
    {"date": datetime.date(2025,7,20), "net_sales": 23.34, "gross_sales": 32.98},
    {"date": datetime.date(2025,7,21), "net_sales": 45.21, "gross_sales": 68.12},
    {"date": datetime.date(2025,7,22), "net_sales": 12.98, "gross_sales": 34.54},
    {"date": datetime.date(2025,7,23), "net_sales": 41.32, "gross_sales": 45.32},
]

df = pd.DataFrame.from_records(data)

def make_figure(df, width=None):
    fig = go.Figure()
    fig.add_trace(go.Bar(
        x=df['date'],
        y=df['net_sales'],
        xperiod=24*60*60*1000,
        xperiod0=df['date'].min(),
        xperiodalignment="middle"
    ))
    fig.add_trace(go.Bar(
        x=df['date'],
        y=df['gross_sales'],
        xperiod=24*60*60*1000,
        xperiod0=df['date'].min(),
        xperiodalignment="middle"
    ))
    fig.update_layout(
        xaxis={
            'ticklabelmode': 'period',
            'tickmode': 'auto',
            'dtick': 24*60*60*1000,
            'range': (df['date'].min(), df['date'].max() + datetime.timedelta(days=1))
        })
    fig.show(renderer="browser")

make_figure(df)
make_figure(df[0:1])

When plotting a bar chart of aggregated timeseries data, strange behaviour happens when there's only 1 data point or group. When there are multiple bars each bar is displayed as one would expect (first screenshot below), but when there's only a single data point the bar is invisible, and specifically the bar width is set to 1ms or 0.5px (second screenshot).

Image Image
Dominant language
Python
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Avg merge
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Merged PRs (30d)
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