The earliest argument map?
From Richard Whately, D.D. Elements of Logic. London: Longmans, Green and Co. 1913. (Originally published in 1826).
This is the only diagram in Whately's Elements of Logic which is close to an argument map. Note that it is an argument map schema or template rather than an argument map. Whately's comment "Many students probably will find it a very clear and convenient mode of exhibiting the logical analysis of a course of argument, to draw it out in the form of a Tree, or Logical Division" suggests that the practice of argument mapping did exist at that time.