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I've got a map showing 26k+ positions. Here is what is shown with a cluster solution :

enter image description here

I'm looking to display differents color/size for different proportions. Above is the minimum zoom when you open the map. You can see the biggest cluster is around 10% of the total positions (2949).

Ofc when you zoom in the cluster is reduced. enter image description here

I'm looking for a "rule" to determine how many types of cluster do I need, what size, ...

But I think the most important point is "How many styles do I set up?" too many colors/size can bore the users (I think?), but, hey, it's almost 30k positions, so the difference has to be shown!

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Not bored, but overwhelmed.

The more colors that are on your map the harder it will be for your user to differentiate what the colors define.

If you take your current map, your user has 3(ish) colors to decipher. Red for a large cluster of items, an orange-yellowish for a medium size of clusters, and a green for a small number of clusters. This is a good scale as it keeps the amount of classes simple. Remember, the idea of clusters isn't to have your user try to decipher what is in each cluster at a high zoom level like the example below. It's to get your user to zoom into the area you deem most important and have them explore more in that area.

Confusing cluster

img source

However if you want to show a difference...

Make the difference visible on the markers when the users get close enough. Don't try to show all these differences in the clusters themselves. You can try something like what is above, but only when you get close enough. It depends on how many different types of markers you have, but I would make sure the user has zoomed in to roughly state level before showing differences of each cluster on the actual cluster.

Remember though, the point is to get users to explore a specific area

Look at this example from Mapbox that shows how while all the clusters look the same, your eye is immediately drawn to the large red cluster in the middle, which compels you to click on it and dig further and further down.

"So what can I do if I want a specific cluster to stand out?"

So let's say you have a specific area where you want to direct your users instead of having them see the differences at a certain zoom level or size like the above example. Maybe it's the largest cluster or maybe there is something of relevance in the area. Either way, you can single these out a few ways. I would consider color, make it it's own marker (so it's a single marker in a see of clustered markers), or (my personal favorite) adding some sort of tooltip on the cluster that says something along the lines of:

Dig down further here to see what was found at Important Event X!

I guess overall though, there is no definitive 'rule' like what you're looking for to tell you what size, shape, color, etc. of your markers. Coming from a cartographic perspective, I'm going to defer up to my first point I made which is the more colors, the harder it can be to decipher. I would say no more than 6 classes of cluster colors.

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    Thank you for that awesome answer. It helps me understand that I don't actually really get all the needs so I can't determine which vizualisation is relevant for all your options of highlighting a specific cluster. I'm keepinng your annotations for a specific zone (towns), I stand on 3 colors but I'm gonna re-scale my proportions, the red one has to be not so common – So4ne Nov 4 '15 at 14:03

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