It seems that this feature – since Apple has changed the way how they display memory usage – isn't a option anymore:
This is no longer an option, especially since Mavericks has new memory management routines (ie, memory compression) that make the charting of memory usage less straightforward. Apple has switched to a new "Memory Pressure" approach to viewing memory usage, which is available in the Memory section of Activity Monitor, but so far has not been added as a Dock icon option. Hopefully Apple will do so, as this would perhaps be one of the more useful Dock icon options.
I've got that from here one would hope that at some point we could graph memory pressure in the dock using Apple's Activity monitor as opposed to needing to open the whole tool with a window that shows many other items.