By default, PDF uses a conventional graph coordinate system, with the origin in the lower left corner. These examples create a 200x200 page with the origin in the middle, a vertical line at x=0, a horizontal lines at y = 0 and text in each quadrant.

page := Graphics.PDF.Page
  newInBounds: (-100 @ -100 corner: 100 @ 100)
  colorspace: DeviceRGB new
  render: [:renderer ¦
    renderer moveTo: -100 @ 0; lineTo: 100 @ 0; stroke.  "line at y = 0"
    renderer moveTo: 0 @ -100; lineTo: 0 @ 100; stroke.  "line at x = 0"
    renderer textObjectDo: [
      renderer setFont: #Helvetica size: 1.
      renderer textMatrix: #(4 0 0 4 50 50); showString: '(50 50)'.
      renderer textMatrix: #(2 0 0 2 -50 50); showString: '(-50 50)'.
      renderer textMatrix: #(2 0 0 2 50 -50); showString: '(50 -50)'.
      renderer textMatrix: #(2 0 0 2 -50 -50); showString: '(-50 -50)']].
page saveAndShowAs: 'demo10_transformationsNone.pdf'

demo10_transformationsnone.pdf

"x -> x  ,  y -> -y
In this example the matrix is coded manually. 
It can also be coded as Matrix scale: 4 @ -4 to get '[4 0 0 -4 0 0]' 
and (Matrix scale: 4 @ -4) translate: 50 @ 50 to get '[4 0 0 -4 50 50]'"
 
page := Graphics.PDF.Page
  newInBounds: (-100 @ -100 corner: 100 @ 100)
  colorspace: DeviceRGB new
  render: [:renderer ¦
    renderer concat: (Matrix scale: 1 @ -1).
    renderer moveTo: -100 @ 0; lineTo: 100 @ 0; stroke.  "line at y = 0"
    renderer moveTo: 0 @ -100; lineTo: 0 @ 100; stroke.  "line at x = 0"
    renderer textObjectDo: [
      renderer setFont: #Helvetica size: 1.
      renderer textMatrix: #(4 0 0 -4 50 50); showString: '(50 50)'.
      renderer textMatrix: #(2 0 0 -2 -50 50); showString: '(-50 50)'.
      renderer textMatrix: #(2 0 0 -2 50 -50); showString: '(50 -50)'.
      renderer textMatrix: #(2 0 0 -2 -50 -50); showString: '(-50 -50)']].
page saveAndShowAs: 'demo10a_transformationsFlipVertically.pdf'

demo10a_transformationsflipvertically.pdf

"x -> -x  ,  y -> y"
page := Graphics.PDF.Page
  newInBounds: (-100 @ -100 corner: 100 @ 100)
  colorspace: DeviceRGB new
  render: [:renderer ¦
    renderer concat: (Matrix scale: -1 @ 1).
    renderer moveTo: -100 @ 0; lineTo: 100 @ 0; stroke.  "line at y = 0"
    renderer moveTo: 0 @ -100; lineTo: 0 @ 100; stroke.  "line at x = 0"
    renderer textObjectDo: [
      renderer setFont: #Helvetica size: 1.
      renderer textMatrix: #(-4 0 0 4 50 50); showString: '(50 50)'.
      renderer textMatrix: #(-2 0 0 2 -50 50); showString: '(-50 50)'.
      renderer textMatrix: #(-2 0 0 2 50 -50); showString: '(50 -50)'.
      renderer textMatrix: #(-2 0 0 2 -50 -50); showString: '(-50 -50)']].
page saveAndShowAs: 'demo10b_transformationsFlipHorizontally.pdf'

demo10b_transformationsfliphorizontally.pdf

"x -> -x  ,  y -> -y"
page := Graphics.PDF.Page
  newInBounds: (-100 @ -100 corner: 100 @ 100)
  colorspace: DeviceRGB new
  render: [:renderer ¦
    renderer concat: (Matrix scale: -1 @ -1).
    renderer moveTo: -100 @ 0; lineTo: 100 @ 0; stroke.  "line at y = 0"
    renderer moveTo: 0 @ -100; lineTo: 0 @ 100; stroke.  "line at x = 0"
    renderer textObjectDo: [
      renderer setFont: #Helvetica size: 1.
      renderer textMatrix: #(-4 0 0 -4 50 50); showString: '(50 50)'.
      renderer textMatrix: #(-2 0 0 -2 -50 50); showString: '(-50 50)'.
      renderer textMatrix: #(-2 0 0 -2 50 -50); showString: '(50 -50)'.
      renderer textMatrix: #(-2 0 0 -2 -50 -50); showString: '(-50 -50)']].
page saveAndShowAs: 'demo10c_transformationsFlipDiagonal.pdf'

demo10c_transformationsflipdiagonal.pdf

"This is the more common layout used for reports"
page := Graphics.PDF.Page
  newInBounds: (0 @ 0 corner: 100 @ -100)
  colorspace: DeviceRGB new
  render: [:renderer ¦
    renderer concat: (Matrix scale: 1 @ -1).
    renderer textObjectDo: [
      renderer setFont: #Helvetica size: 1.
      renderer textMatrix: #(2 0 0 -2 10 10); showString: '(10 10)'. 
      renderer textMatrix: #(2 0 0 -2 10 20); showString: '(10 20)'.
      renderer textMatrix: #(2 0 0 -2 10 30); showString: '(10 30)'.
      renderer textMatrix: #(2 0 0 -2 10 40); showString: '(10 40)'.
      renderer textMatrix: #(2 0 0 -2 20 10); showString: '(20 10)'. 
      renderer textMatrix: #(2 0 0 -2 20 20); showString: '(20 20)'.
      renderer textMatrix: #(2 0 0 -2 20 30); showString: '(20 30)'.
      renderer textMatrix: #(2 0 0 -2 20 40); showString: '(20 40)']].
page saveAndShowAs: 'demo10d_transformationsFlipVerticallyTranslateOrigin.pdf'

demo10d_transformationsflipverticallytranslateorigin.pdf