How do graphic objects in data visualisation facilitate ease of interpretation? Background: I vaguely remember reading in a book (I think it may have been Nudge - Thaler and Sunstein) about the advantages of using graphics for visualising data, such as a smiley face, or traffic lights, to communicate a message in an easily interpreted, visual language. 


To what extent do graphics such as smiley faces and traffic lights in visualisations facilitate ease of interpretation?
What is the cause of this ease of interpretation?
What research has studied this phenomena?


Initial thoughts: 
I imagine that the ease of interpretation is due to these items being purely visual that we quickly recognise them, and they can make complicated, unfamiliar information (for instance health results) that much easier to comprehend. 
Is that an accurate summary? 

If it's any further help I'm looking into this specifically for use in the communication of health results. So for example if you have a 20% risk of developing a disease, a good way to illustrate this could be 100 smiley faces - of which 20 are :( and 80 are :)
As you mentioned, data visualization and visual stimulation in general, trigger a primitive brain response, which results in an easier comprehension and engagement/rejection process - a process more trivial than sensing and comprehending symbols. Thus, numerical information such as percentages, distributions, and relativistic data in general, immediately get infused into the cognitive realm, provided the visual field is not excessively cluttered. Too much visual stimulus in a presentation results in a more laborious attempt at comprehension, and hence may lose favor.

When it comes to non-numerical / non-quantitative data, visualizations serve as a promoter of cognitive engagement. Traffic lights to draw attention, smiley faces may be used to encourage the receiver, boxes and other demarcations to engrave specific items of information in memory, the list goes on. 

I think the effectiveness of the visual presentation largely depends on the creativity and imagination of the presenter to resonate with the target audience, in terms of the mood, awareness, humor (mood?), social values and other variables which maybe of interest, of the target audience.

Again, if the visual field is excessively populated, the labor involved in the comprehension process may impede the effectiveness of the presentation. 

Also, the ease at which visual information is processed also depends on how distinct the objects that are communicating information in the visual are.
In your example, smiley faces to depict percentages maybe less effective compared to, say a colored pie chart, or something to that effect, because smiley faces are not easy to distinguish by themselves - even if they are ordered into separate regions. And in the possible event of conveying grim news , grim graphics may not received in the highest of spirits :P. 

Cheers!