Sum of graphs of continuous functions and boundedness of additive operators

Assume that f:D1→R and g:D2→R are uniformly continuous functions, where D1,D2⊂X are nonempty open and arc-connected subsets of a real normed space X. We prove that then either f and g are affine functions, that is f(x)=x∗(x)+a and g(x)=x∗(x)+b with some x∗∈X∗ and a,b∈R or the algebraic sum of graphs of functions f and g has a nonempty interior in a product space X×R treated as a normed space with a norm ‖(x,α)‖=‖x‖2+|α|2.