To get even this far, readers have to make great efforts to join the dots.
I encountered this sentence in The Economist. I have looked up the dictionary but still cannot understand the meaning of "join the dots". Could you please help me with that?
The whole paragraph is
The author seems to be suggesting that tacit mass collusion in history’s greatest crime turned Germans, through fear of their own looming retaliatory victimisation, into fanatics. But this grand hypothesis emerges only in fragmented form from these individual accounts. To get even this far, readers have to make great efforts to join the dots. Many will be left yearning for more help from the author.