13.7.1  Lines and directed lines in the plane: line

See Section 14.5.1 for lines in space.

The line command returns and draws a directed line. It can take its arguments in different ways.

Two points:

• line can take two arguments:
  P,Q, two points (which can also be given as a list).
• line(P,Q) returns and draws the line whose direction is from the P to Q.

A point and a slope.

• line can take two arguments:
  • p, a point.
  • slope=m
• line(p,slope=m) returns and draws the line through the given point with the given slope, where direction of the line is determined by the slope.

A point and a direction vector.

• line can take two arguments:
  • P, a point.
  • [u₁,u₂], a direction vector.
• line(P,[u₁,u₂]) returns and draws the line through the given point with the direction given by the direction vector.

An equation.

• line can take one argument:
  a*x+b*y+c=0.
• line(a*x+b*y+c=0) returns and draws the line given by the equation. The direction of the line is given by [b,−a].

Example.
Input:

or:

or:

or:

Output:

Warning: To draw a line with an additional argument for color (such as color=blue), this argument must be the third argument. In particular, for a list of two points to specify a line in this command, the list must be turned into a sequence, such as with op. For example, given a list L of two points (possibly the result of a different command) which determines a line, to draw the line blue enter line(op(L),color=blue); entering line(L,color=blue) will result in an error.