[Click on the picture for full size]
This illustration is by no mean the be-all-end-all of sonic guideline, but it could serve as a quick and easily understandable visual reference for novice to pick a sound signature that is more subjectively preferable (and can be referenced back to my Concise Multi-IEM Comparison) No IEM is perfect, and certainly no IEM can have everything (even though a very few do get close). The idea is to start with one of the six sound signature that is most preferable to you, then look for other traits that could fit into your taste.
For example, if you like a bassy sound, then the two other possible choices besides a purely bass dominating sound are 'balance' and 'warm+sweet', depends on whether you like your bass with more mid or more treble. However, the likelihood that you will enjoy an 'analytical' is probably close to zero. A 'bright vocal' is a long shot as well, but a 'mid centric' sound might be better than none, if you have run out of other choices.
Some of these sound are naturally the opposite of each others, while other might not be considered as a direct path of upgrade. So it is more likely you will find an agreeable sound on the neighboring signature than those that are on the other sides. For example, you can't have huge soundstage with sweet, fully intimated vocal - they just don't match up that way. Either you are getting one over the other, or they will compensate for each other (and both come up short). These are the relationship that this graph is trying to show you.
No graph has even been able to fully illustrate the complex relationship of sound signature and personal preference, and this graph is no exception. So take it as an interesting opinion, and nothing more. Hopefully it could help you simplify your choice, but never takes it as absolute truth.
Note: Bright Vocal is labeled the 'missing link' because IEM with such a sound is more likely to get categorized as either analytical or mid centric rather than being referred as a sound signature of its own. It is also a rare signature and often not fond by most.