This is an example of the syntactic device called “pied piping”†. In this, a relativizer (in this case which) standing for the object of a preposition in the following relative clause drags the preposition along with it to the front of the clause so that the syntactic roles are more clearly visible.
In your example, the relative clause headed by which is additional information about the line which can be represented as a ‘canonical’ independent clause:
To make this a relative clause modifying the noun phrase An imaginary straight line &c, we replace ‘the line’ with which and move that to the front of the clause:
But in a longish clause there is some danger that the reader may lose track of the relationship between which and about; the reader may see about to sitting together and read that as meaning “on the verge of”, which is pretty nonsensical. To prevent that sort of misparsing and ensure that the reader understands that which is the object of about, we “pied pipe” the preposition along with its object:
† The term derives from a charming German story made famous in the English-speaking world by Browning’s The Pied Piper of Hamelin.