The --graph option runs Drake in "graph" mode. Instead of executing steps, Drake just draws a graph of all the inputs and outputs in the workflow, with color-coding to indicate which will be run.

The graph is saved to a file named drake.png in the current directory. Files which will be built are colored green, and those which were forced will be outlined in black as well.

You can specify --graph=dot to output a .dot file instead of outputting a png, or --graph=show to display the image on screen.