Perspective calibration#

Use Perspective calibration when a graph has been photographed at an angle or its plot area becomes visibly narrower towards one side.

How to use it#

1. Choose Perspective.
2. Select Set corners and place the four corners of the graph’s plot rectangle.
3. Use Edit corners to refine them if needed. The optional grid helps confirm that the geometry follows the plotted area.
4. Enter the X values at the left and right edges, and the Y values at the bottom and top edges.
5. Set Linear or Logarithmic scale independently for X and Y, then add optional labels and units in Axis details.

Fast calibration is sufficient when the graph is already front-facing. Perspective calibration is intended to correct genuine projective distortion rather than ordinary rotation.

Technical details#

GraphX uses a projective mapping (a homography) between the rectangular graph coordinate system and the four-corner image geometry. In simplified form, a homogeneous coordinate is transformed as:

The final image position is obtained by dividing by $w'$. The practical result is that calibration lines can follow a photographed plot even when its edges are not parallel in the image.

For the rectified coordinate system and inverse mapping, see Calibration mathematics.