In a different location Mark does talk about the x-axis - the x-axis is "Time and Money"
http://smartfaststartup.com/2011/06/08/real-secret-to-startu...
Which make the loops palatable.
http://smartfaststartup.com/2011/06/08/real-secret-to-startu...
Which make the loops palatable.
"He was saying a real startup trajectory might look something more like this" (emphasis mine)
When we're graphing trajectories, not functions, loops are certainly allowed (see e.g. phase diagrams[1]). Still, without labeled axis those graphs are pretty much meaningless, so I treat them more like an conceptual illustration.