While you could use the tag to locate and work with graphics obejcts, actually the simplest and most reliable way to access graphics objects is to use handles. All graphics objects (lines, patches, axes, images, etc) are represented by handles, and you can pass those handles to other functions or use the handles to set or get any of the properties of that object. To know how to access graphics objects using their handle refer to the MATLAB documentation:
Graphics functions (except for some high-level ones) return a handle to the object that the function creates. You simply need to allocate this handle to a variable and use it as required. I would recommend that you always obtain and use all graphics object handles explicitly, and never rely on gca or gcf etc in code that you actually want to work reliably.
Here is a simple example of using graphics handles:
fgh = figure();
axh = axes('Parent',fgh);