You can save graphs on the Dashboard pages in either .csv format (the raw data for the graph) or .png format (an image file showing the rendered graph). Where graphs are displayed on the Monitor, Graphs, and Query Analyzer pages, 2 icons to the right of the graph title let you export in these formats.

Hover over the icon to confirm which one corresponds to which format. Since .png files typically display in the browser rather than bringing up a Save dialog, either click the icon and save the image file on the resulting page, or bring up the context menu for the icon and choose Save linked file or the equivalent for your browser. (You might need to change the extension to .png in the Save dialog.)

For .csv files: