Your best bet is to convert the chart to a .png image and import the image file into PDF. There is a code example
here that shows one way to convert a chart to an image. It only works reliably in Chrome, so I would not suggest using it on a production website, but it should suffice for converting your charts to image files for personal use.
Alternatively, you could wait for image conversion to be added to the API. The developer team has hinted that it is coming soon.