plotly / plotly/plotly.py

[BUG] Data in `px.imshow` disappears at default zoom, reappears when zooming in with dense data

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bug P2
Dominant language
Python
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Description

Environment
Plotly Version: 6.1.2
Operating System: Windows 10 22H2 19045.5854 Windows Feature Experience Pack 1000.19061.1000.0
Browser: Chrome 137.0.7151.41

When trying to visualize dense binary time-series data (e.g., 10 channels over 4,000+ time points), both px.imshow and px.timeline fail to render the data correctly in the default, zoomed-out view. The areas that should contain data appear blank.

However, if you manually zoom into any region of the plot, the data correctly appears. Resetting the zoom causes the data to disappear again. This suggests the issue is not with the data itself, but with the rendering engine's handling of very thin graphical elements (heatmap cells or gantt bars) at a low zoom level.

Here is my origin data.
BAO_spikes_sorted_output.csv

Here is my html.(Please change the suffix to html)

BAO Spike Heatmap.txt

Here is my code.

def plot_binary_heatmap(csv_file="./data/down/combined_binary.csv", title="Binary Heatmap of Signals"):
    """
    Reads data from a combined binary CSV, processes filenames, and plots a heatmap.
    The x-axis represents time in seconds, and the y-axis represents file numbers in ascending order.
    """
    df = pd.read_csv(csv_file)

    # 1. Extract the time column (keep as float)
    if 'Time' in df.columns:
        time = df['Time'].astype(float)
        df = df.drop(columns=['Time'])
    elif 'time' in df.columns:
        time = df['time'].astype(float)
        df = df.drop(columns=['time'])
    else:
        # If no time column exists, create an index
        time = pd.Series(range(df.shape[0]))

    # 2. Transpose: each row is a file, each column is a time point
    df_t = df.T
    df_t.index.name = "File"

    # Update index, keep only the part before "_"
    df_t.index = df_t.index.to_series().str.split('_').str[0]

    # 3. Set time as column names (keep as float)
    df_t.columns = time

    z = df_t.to_numpy()
    y_labels = df_t.index.tolist()
    x_values = df_t.columns.to_numpy()

    # 4. Create the heatmap
    fig = px.imshow(
        z,
        labels=dict(x="Time (s)", y="Channel", color="Binary"),
        x=x_values,
        y=y_labels,
        color_continuous_scale=["#ffffff", "#000000"],
        aspect="auto",
        title=title
    )

    # 5. Update chart layout
    fig.update_layout(
        xaxis_title="Time (s)",
        yaxis_title="Channel",
        height=600,
        width=1000,
    )

    fig.write_html(f"{title}.html")

plot_binary_heatmap(csv_file="BAO_spikes_sorted_output.csv", title="BAO Spike Heatmap")

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First steps

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  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Start by reproducing the report with the attached BAO_spikes_sorted_output.csv, the provided Python px.imshow example, and Plotly 6.1.2, comparing the default view with a zoomed view. Done means dense binary heatmap data remains visible without manual zooming and the behavior is verified against the supplied case.

Written by the indexing model from the issue text.

Assessment

Tech stack
plotly, python
Domain
data-visualization
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Needs clarification
Newbie friendliness
35/100

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