So my response was: "I know 'grey area' is a figure of speech, but if you are going to talk about a grey area, it is very helpful to your audience to show that in grey"!
Meanwhile, what has become of decent timelines? Increasingly I find presenters saying: "Here is the timeline" and they show me a bunch of bullet points running from the top of the screen to the bottom. There is not a line in sight!
In these cases I explain that timelines emerged because they can be very effective at displaying a progression over a fixed period and for spreading the elements appropriately across that period, in comparative proportions. I then go on to explain that the best timelines make their big point 'at a glance' without the reader or audience even having to study any accompanying detail.
Here, for instance, is a nice twist on the classic Human Evolution timeline where you instantly see the point that our activity levels and posture are beginning to regress:
Similarly, a look back at how mobile phones became smaller and smaller, then began to grow back to a size - front-on at least - approaching that of their starting point:
It's always useful to keep reminding yourself that, while you are very familiar with all that you are saying and displaying, your audience will probably be hearing it and seeing it for the first time. So, to ensure maximum clarity it all needs to be a bit simpler than you might think. You are never going to achieve that if your words are out of sync with your visuals!