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,
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
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.