The document is wrong
Charts and canvas elements are blank in the PDF
A correctly sized empty rectangle where the chart should be. The axis labels are sometimes there and the plot area never is.
What is actually happening
A canvas element draws imperatively. If the drawing call has not run by the time the page is captured, the canvas is a blank bitmap of the right dimensions, which is exactly what you are looking at. SVG-based charts do not have this problem because the shapes are in the DOM.
Confirming it is this and not something that looks like it
If the chart library can output SVG, switch it for one render. If the chart appears, the cause was the canvas timing and not the data.
The fix
Render the chart to SVG on the server and put the markup in the document. Most chart libraries have a server-side or static rendering mode for exactly this, and the output is vector rather than a bitmap, so it stays sharp at print resolution.
If that was not it
These produce the same symptom often enough to be worth ruling out before assuming the fix above did not work.
- A chart with an animation duration, where the render catches it partway through
- A canvas sized by JavaScript from its container, which is zero-width before layout settles
- WebGL charts, which need a GPU context that a headless render may not have
- Chart fonts loaded separately, which shift labels after the snapshot
Frequently asked
Does this happen with every rendering engine?
The behaviour behind it is not specific to one tool. A canvas element draws imperatively. Anything rendering HTML to a paged medium has to make the same decision, so the fix travels with you if you change how the render happens.
Will this show up as an error in my logs?
No, and that is what makes it expensive. A document that renders wrong is still a successful render as far as every log line is concerned. It is found by someone opening the file, which is usually the customer.
Related failures
Problems people arrive at from the same starting point, or mistake for this one.
The generated PDF is blank
Almost always one of three things: the content is rendered by JavaScript that had not run when the snapshot was taken, the body has no height because everything inside it is absolutely positioned or floated, or the markup you sent was empty because the template call returned before the data arrived..
Background colours and images are missing from the PDF
Browsers drop backgrounds when printing, because a page with every background painted would empty a toner cartridge, and most render APIs inherit that default.
Images do not appear in the PDF
The image src is a path rather than a URL.
The PDF ignores my CSS
The stylesheet is linked rather than embedded, and the link points somewhere the renderer cannot reach: a bundler-generated path, a digested asset filename, or a URL behind your application's authentication.
Content rendered by JavaScript is missing from the PDF
The snapshot was taken before the script finished.
CSS for paged documents, and what actually works
Which parts of the paged media specification a browser-based render implements.
Everything that goes wrong, by category
The full list, grouped by where in the pipeline it breaks.
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