Contacts seem to be missing from my maps (what's "no data"?)

Contacts may appear to be missing from your maps if certain groupings are selected.

This is best explained by example:

We can see here that we have one contact in the USA.

However, if we group by Industry the pie chart reports "No data"

This has happened because our contact in the USA doesn't have an industry assigned to them. As such North America has one contact but there is "No data" to allow us to group by Industry.

In contrast, if the whole continent doesn't have any contacts then the pie chart will report "No contacts"