Chapters: 1: Introduction 2: Simple example 3: Invocation 4: Finer Control 5: XY Plots 6: Contour Plots 7: Image Plots 8: Examples 9: Gri Commands 10: Programming 11: Environment 12: Emacs Mode 13: History 14: Installation 15: Gri Bugs 16: Test Suite 17: Gri in Press 18: Acknowledgments 19: License Indices: Concepts Commands Variables 
9.3.8: `

`differentiate {xy wrt indexyx}  {grid wrt xy}' 
Differentiate column data or grid data. Only the `x
' and `y
'
columns may be differentiated. They may be differentiated either with
respect to (``wrt'') the index (forming a first difference) or with
respect to the other column. The derivative is done with the
backwardsdifference algorithm. Grid data may differentiated with
respect to `x
' direction or `y
' direction. Grid
differentiation is done with a centred difference, with endpoints being
assigned the derivative of the neighboring interior point (so that the
second derivative is zero at the edges of the grid).