Charts, maps, photographs, and working drawings
Some pictures are not of a thing you could point at. They are of a quantity, a relation, or a place, reduced until a reader can hold it. A graph or a chart is a graphic that represents tabular or numeric data. Charts are used because a large set of numbers is hard to read as numbers, and easier once the relationships between parts of the set are drawn. The drawing can lie, of course. A chopped axis will do it. The form itself is not the lie. The form is a way of letting a relationship be seen at all.
Diagrams, symbols, maps
A diagram is a simplified and structured visual account of concepts, ideas, constructions, relations, statistical data, and so on, used to make the topic clearer by showing it. "Simplified" is doing a lot of work in that sentence. A diagram that refuses to leave anything out has stopped being a diagram and become a dump. The skill is knowing which relation is the one the reader came for.
A symbol, in the basic sense, stands in for a concept or a quantity: an idea, an object, a quality. In a more philosophical register, concepts themselves are symbolic, and the marks we make are tokens, allegorical to a meaning they do not directly encode. You do not need the philosophy to use the fact. A north arrow, a resistor in a circuit, a red cross, a play button: each is a small graphic that works because a group of people has agreed what it stands for. Break the agreement and the mark is only a shape.
A map is a simplified depiction of a space, a navigational aid that picks out relations between objects in that space. Usually it is a two-dimensional, geometrically careful representation of a three-dimensional space. The care is the product. Leave out the geometry and you have a sketch of a route, which is sometimes what you wanted and sometimes a way to get lost politely. Martin Waldseemüller made one of the first maps that still look modern to us, a printed world picture with a coast a reader could treat as information rather than as ornament.
The photograph
One difference between photography and other graphics is that a photographer, in principle, just records a single moment, with seemingly no interpretation. The "seemingly" matters. A photographer chooses the field of view and the angle, and may use other techniques: a lens to choose the view, a filter to change the colours. Digital photography then opened an indefinite number of fast, strong manipulations. The negative, when there was a negative, was already not innocent. Shifting the viewer's attention with pinpricks in the emulsion could change what a picture seemed to say.
Even early on there was an argument about photographs of enacted scenes presented as real life. War photography is the hard case, because the original event is often impossible to record as it happens, and a picture made a minute later, or a picture staged, can be offered as if it were the minute itself.
The field of view has a strong effect. It censors the rest of the scene, either by cropping it afterwards or by never including it. That brushes against the question of what the reality of a scene even is. The human brain handles incoming information with the help of what it already expects, so people see what they are prepared to see. Photography does a version of the same thing. The photographer interprets the scene for the viewer, whether or not the caption admits it.
The working drawing
An engineering drawing is a drawing of a technical kind, used to define the requirements for a made thing fully and clearly. It is usually produced to standardised conventions: layout, names, how a line is to be read, type, line styles, sizes. Two draughtspeople who have learned the same conventions can argue about a part without the part being in the room. First-angle projection is one of those conventions. It fixes where the front, the top, and the side sit on the sheet, so that "left" on the paper means a particular left on the object.
From here the step into the machine is short, and it splits in two. There are two types of computer graphics. Raster graphics define each pixel separately, as a digital photograph does. Vector graphics use mathematical formulas to draw lines and shapes, which are then interpreted at the viewer's end to produce the picture. Vectors give a sharp result at any size and often a smaller file. When the picture is complex they can take time to draw, and the file can end up larger than a raster of the same subject. The working drawing is where a lot of people meet it first, because a plot of a bracket is already a vector picture whether or not anyone uses the word.