Marks on the graph can be any image available on the web; you need to use the full URL
(mark=http://i.cdn.turner.com/cnn/.element/img/3.0/global/red_bull.gif to use one of CNN's icons for the datapoints,
or label-mark=http://bililite.com/images/silk/add.png 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=http://bililite.com/images/silk/clock.png.
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.
