As I understand it, a diet can be broadly described in term of macronutrient ratios—i.e., the proportions of energy received from carbohydrate, protein and fat sources.
In that case, presumably the range of all possible diets can be plotted on a ternary graph, ranging from (33,33,33) (i.e., a completely balanced diet) to (100,0,0) / (0,100,0) / (0,0,100) (i.e., a diet in which all energy comes from a single type of source).
What would such a map of the ‘dietspace’ look like? Where would the healthy and unhealthy regions be? What regions would come with unexpected side-effects, positive or negative (such as ketosis maybe helping to treat epilepsy)?
I've had a go at making a rough version, but as you can probably tell I am neither an artist nor a dietician (the dashed lines show overlap, which is difficult to get across in Paint):
