Question: I have some awful problems with editing picture and table captions in a paper I'm working on. Maybe some native speaker could give me a hand. One of the captions is for example:

Figure 14. The dynamics of crime in X. **A** causal chain.

It's under a diagram with which I want to explain the dynamics of crime in X - the diagram presents a causal chain.

Should it be "A causal chain" or just "Causal chain"?

Answer: 
OK then -- following your clarifications, I suggest these possibilities:

> 
> Figure 14. The dynamics of crime in X: a causal chain.
> 
> 
> 

(As a typical example of such a chain)

> 
> Figure 14. The dynamics of crime in X: the causal chain.
> 
> 
> 

(Refers exclusively to the causal chain in X).
To be clearer, state the relationship between "crime in X" and "causal chain" in your caption, and use a full sentence. For example:

> 
> Figure 14. The dynamics of crime in X represent a causal chain.
> 
> 
> 

or

> 
> Figure 14. A causal chain showing the dynamics of crime in X.
> 
> 
>