Marks on the graph can be any image available on the web; you need to use the full URL (mark= to use one of CNN's icons for the datapoints, or label-mark= to use one of the icons stored on this site for the label lines).

Rather than try to draw my own markers for the graphs, I am hosting the amazing icon set called Silk, by Mark James, and a few similar sets. Again, use them with the full URL, like mark=

If you don't like these, Google Charts API lets you create many more dynamic icons. Don't forget to URLencode the URL since it includes & characters that won't work otherwise. Or at least change the & into %26.

By Mark James. Under a Creative Commons Attribution 2.5 License