No, not an additional pie chart (but actually yes, see * below)
Currently, I always see a large block called "unknown", despite having vision on enemy cities, which are thus known!
Example:Assume a map with 20 cities, blue owning 7 (35%), red owning 6 (30%), green owning 4 (20%) and 3 (15%) remaining neutral.
The red player currently has vision on 2 blue cities, obviously all his own cities, all 4 green and 2 neutral cities. Thus 6 cities are hidden through the fog of war.
Currently, the red player is given a pie chart saying 30% and 70% unknown. This is not very helpful, as the red player has much more knowledge.
Instead, the chart should show: blue 10%, red 30%, green 20%, neutral 10% and 30% unknown. This reflects more of the players actual knowledge about the game. Knowledge that a player already has would thus become more easily available.
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The above was my original suggestion, but it might be improved further. I don't think that the suggestion below is difficult to implement either, since essentially one need a single loop over all cities, counting their status.-----------------------------
* However, the player does know even more: the map displays cities hidden by the fog of war with the status that was last seen by the player. Therefore an additional pie chart** would be useful, which includes this insecure, but often quite probable information.
Example:In the above example, 6 cities were hidden from the red player by the fog of war. Red's map shows 3 of these as neutral (they were never scouted by red) and 2 are shown as green (unbeknown to red, these two were lost to blue meanwhile) and 1 city within the fog of war is shown as blue.
So the pie chart that includes unsecure/outdated scouting would show:
blue 15%, red 30%, green 30%, neutral 25%.
Of course, relying on unsecure/outdated scouting information might be misleading, as this example shows; but often enough it is quite reliable.
It is of course an essential part of the players task to judge the actual game situation correctly! However, given both a pie chart with the currently scouted situation as well as a pie chart including outdated/insecure information would make this part of a player's task less tedious, since it is quite tedious to count city dots on the minimap!
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** Of course, one could combine actual and outdated scouting in a single pie chart, e.g. using a bright shading of a player's color for "currently in vision" and a dark shading for "seen, but currently within fog of war and thus unsecure", but I think this might be confusing too some players.
Example:In the above example, such a combined chart for red would then show 10% bright blue, 5% dark blue, 30% bright red, 20% bright green, 10% dark green, 10% bright grey and 15% dark grey.
I am not sure whether a combined chart is clearer to read. If there is enough space, comparing both charts might be better. I guess this is a personal preference, you could make it a gold member option to choose between combined and separate pie charts?! After all, these improved charts only provide some comfort, but not anything a player could not find out by spending a minute to count city colors on the mini map.
However, I think a much better option would be to always have separate pie charts: free members are shown the chart with current scouting information, and with a 5% chance they also see the pie chart with the insecure/outdated information (and a greyed out are otherwise). 20% for silver members, 100% for gold. So players get a taste of this feature and may close and reopen the game objective dialogue repeatedly in order to see both pie charts at once - or improve their membership level.