Element with No Proton?
A free neutron is not considered to be an element and there is no element that does not have at least one proton.

Free neutrons could be included on the graph as they are unstable and decay with a half life of around 10 minutes ($611.0\pm1.0~\mathrm{s}$) although most versions I have seen do not usually include them as they aren't really nuclei which is what the graph is about.