> How did they explain what plotting was without defining a line?
Most plots aren't lines, so there's no reason this task would require defining a line. In fact, the shape formed by a graph is called a "curve" for just this reason.
Interestingly in Elements that is a line: a line was defined only by having no breadth (= being 1-dimensional). What is now generally called a line would have to be qualified as straight.
