There are two concerns when allocating screen real-estate to user interface elements: Legibility, and Tangibility.
Legibility is the screen-space needed for the user to be able to recognize what information an element is trying to convey. Text is the most obvious example. Graphs will be different. Simpler representations require less space. A dot could be less than a millimeter in size, whereas text needs to be at least a few millimeters tall.
Tangibility is the screen-space needed for interactive controls to remain an easy pointer target for the user. The unit size for a control to be tangible changes based on the pointer device. A touch screen needs at least a centimeter, whereas a stylus or mouse require less.