When most people think of the gender spectrum, they think in terms of blue and pink, and maybe some purpley stuff. That model isn’t much of a spectrum and is rather a scale of femininity vs masculinity, when in reality gender is only sometimes on those terms, and is a huge, multi-dimensional concept that simply can’t fit on a single line. At this point, I would like to propose the term "gender sphere” rather than “gender spectrum” to emphasise its absolute complexity.
EDIT: There is much confusion about the implications of the axises of the Gender Sphere. I promise I am not trying to rewrite genetics (even though gender =/= genetics anyway). It is very simply the depiction of a 3D space on a 2D surface. Nothing more.