def draw
  background(200)
  fill(0)

  x1 = 10
  y1 = 90
  x2 = mouseX
  y2 = mouseY

  line(x1, y1, x2, y2)
  ellipse(x1, y1, 7, 7)
  ellipse(x2, y2, 7, 7)

  # d is the length of the line
  # the distance from point 1 to point 2.
  d = dist(x1, y1, x2, y2)

  push do
    translate((x1 + x2) / 2, (y1 + y2) / 2)
    rotate(atan2(y2 - y1, x2 - x1))
    text(format("%.1f", d), 0, -5)
  end
end