I have a copletley retarded theory but hear me out. stop thinking of coordinate systems as a two dimesional system. Imagine a sphere which is infinetly large with two rings on it as wide as possible in a 90° to eachother, this will be our x and y axes. 0 at the start and half way around the negative and positive values intersect. This would make it possible for a tangens function to be drawn without lifting the pen from the paper(more like sphere in this manner). Also for small values (beneath infinte) it would work just as a regular coordinate system beauce we can ignore the cuveture from the z axis if we watch it from the view of a spectator above.