Apparently, diagrams in dashboards are not reading the boards data. They are reading a lazily updated copy of the boards data.
Problems are:
1) _the copy is updated too lazily_, so we won't have a chance to see whether any change in the originating board gets reflected in the graphs.
2) Changes in the names of the columns are not read, except if there wasn't any card in a given state at the moment of the original import. Changing names or colours of statuses only affects newly appeared cards. The result is a mixture of past and present names. :-/