Re: How to shade and interpolate a scattered data file ?

[Mick Spillane <spillane@xxxxxxxxxxxxx>]

Hello Ding Chi,

You are right in thinking that shade (also fill and contour) apply to
data that are 2-D with an underlying grid.  The command "define axis"
is used to define the grid axes, not the plotting region.  If your data
are 2-D but scattered, you might try the "polygon" command if you want
to avoid interpolating the scattered points to a grid.  Here is a simple
example where I generate random x-z locations and define the value to
be represented as an algebraic function. At each point a rectangular
box is filled with a color that represents the value.  The size of the
boxes is controlled by the values dx and dz

region/i=1:1000 ! 1000 data triplets lon,depth,value
! define the data triplets
let xx=i
let/title="Longitude" lon=10*randn(xx)
let/title="Depth" depth=(-50)*randu(xx)
let value=cos(0.01745*lon)*exp(depth/10)
! and the corners of the box centered on each data point
let dx=0.5 ; let bx=ysequence({`(-1)*dx`,`dx`,`dx`,`(-1)*dx`})
let dz=0.5 ; let bz=ysequence({`dz`,`dz`,`(-1)*dz`,`(-1)*dz`})
! then use "polygon" to show the data structure without interpolation
polygon bx+lon,bz+depth,value

Hope this helps.  In your case you would read in the data triplets with
something like
file/form=free/var=lon,depth,value your.file
let dx=... ; let bx=ysequence({`(-1)*dx`,`dx`,`dx`,`(-1)*dx`})
let dz=... ; let bz=ysequence({`dz`,`dz`,`(-1)*dz`,`(-1)*dz`})
! where dx and dz are values that make suitable sized boxes for your case
polygon bx+lon,bz+depth,value

One slight change you might need is if your depth values are all positive
(say in the range 0 to 50) and you want the deepest values at the bottom
of the plot.  Then you would use something like

polygon/vlimits=50:0:-10 bx+lon,bz+depth,value

Good luck,
Mick

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