altair: chart.save does not support pdf as shown in docs

The docs for saving charts show chart.save('chart.pdf') as one of the options, but running that results in ValueError: unrecognized format: 'pdf', even though altair_saver is installed and from altair_saver import save; save(chart, "chart.pdf") works.

By contrast, chart.save("chart.svg") works fine.

Looking at the docstring of Chart.save, it looks like pdf isn’t even supposed to work: “Supported formats are json, html, png, svg”. But maybe it’s just a matter of wiring it up?

If the issue is more complicated than that (maybe because Chart.save is currently hardwired to use the Selenium backend, which doesn’t offer pdf export?), then I would suggest just removing chart.save('chart.pdf') from the code example and refer users who are interested in pdf export functionality to altair_saver’s docs 😃