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Test graph PDF output with more than 1000 points #3988

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Test graph PDF output with more than 1000 points #3988

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test: cover graph PDF output at large point counts
lorenzozanee Sep 20, 2026
78b0d7d
Merge branch 'dev' into fix/graph-print-large-data
camdecoster Sep 21, 2026
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36 changes: 36 additions & 0 deletions components/dash-core-components/tests/integration/graph/test_graph_print.py
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import base64

import pytest

from dash import Dash, dcc


@pytest.mark.parametrize("point_count", [1000, 1001])
def test_grpr001_graph_survives_print(point_count, dash_dcc):
app = Dash(__name__)
points = list(range(point_count))
app.layout = dcc.Graph(
id="graph",
figure={
"data": [{"type": "scatter", "mode": "lines", "x": points, "y": points}],

@T4rk1n T4rk1n Sep 23, 2026

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type: "scatter" never goes down the path that breaks. 1000 and 1001 points both render as SVG, so the parametrize doesn't test the threshold at all. The switch at 1001 happens because px picks scattergl. Can you use scattergl (or build the figure with px.line) so the test hits the WebGL path?

"layout": {"width": 600, "height": 400},
},
)

dash_dcc.start_server(app)
dash_dcc.wait_for_element("#graph .main-svg")

driver = dash_dcc.driver
if not hasattr(driver, "execute_cdp_cmd"):
pytest.skip("print-to-PDF regression requires Chrome DevTools Protocol")

driver.execute_cdp_cmd("Emulation.setEmulatedMedia", {"media": "print"})
pdf = driver.execute_cdp_cmd(
"Page.printToPDF", {"printBackground": True, "preferCSSPageSize": True}

@T4rk1n T4rk1n Sep 23, 2026

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The issue says Chrome's native print works fine at 2000 points. The failing cases are Firefox print and html2pdf/html2canvas. A Chrome Page.printToPDF check passes whether the bug is there or not. What case is this meant to fail on?

)

pdf_bytes = base64.b64decode(pdf["data"])
assert pdf_bytes.startswith(b"%PDF")

@T4rk1n T4rk1n Sep 23, 2026

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These assertions don't look at what got printed. Any page makes a PDF with a %PDF header that's over 1000 bytes, and the is_displayed() check reads the live DOM, not the PDF. An empty plot in the PDF (the reported symptom) passes all three. If you want to test print output, can you render the PDF, or a canvas snapshot of the graph, and check that the trace pixels are there?

assert len(pdf_bytes) > 1000
assert dash_dcc.find_element("#graph .scatterlayer .trace path").is_displayed()
assert dash_dcc.get_logs() == []
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