Should figure captions include the "take home message" that the figure conveys? Should figure captions only describe all of the necessary components (e.g. the meaning of axes, etc.) or can the figure caption also include a brief sentence about the main conclusion drawn from the figure. For example, say the axes are labelled "speed" and "time". One caption might be "Speed of a giraffe in meters per second as a function of time in seconds from when it first starts moving." It could also have the additional sentence "... Note that Speed is increasing with time." For the previous example, this is probably unnecessary, but for more complicated "take home messages" the point of the figure may not be as obvious. So should comments describing the main result behind the figure be only put in the text body or is it OK to put it in the caption as well? I think this might be field dependent.

The justification for wanting to put the main message of a figure in the caption is that it makes the paper easier to skim, as, anecdotedly, I've heard quite a few people say they skim the paper and focus most of their effort on the figures before ever looking at the results.
A short comment in a caption is ok. But generally captions should stay descriptive of what is drawn in the figure e.g. 


  Fig. X: estimated airspeed velocity of an unladen European swallow. 


They should also ideally not be too redundant with legends although sometimes it helps to say things like "shown at various headwind velocities". The place to draw attention on a specific aspect of the data shown in a figure is the results section.

These are customs and not strictly-enforced rules, nothing bad will happen to you if you take a few liberties. Worst case scenario, you'll have to make a few minor edits during revision.