3 argument shade, sigma coordinates, layer depths

["Hein Zelle" <zelle@xxxxxxx>]

Dear Ferret users,

I've spent some time trying to make plots of sigma coordinate data as
shown in the example in the FAQ, using the 3 argument shade command. I
want the layer interfaces to be plotted at the right depth, and the
layers should have solid colors. I have it working almost correctly,
except for the depth of the layer interfaces. The problem is that my
depth is defined as layer thickness. I create x_page and y_page
variables as follows:

! copy x coordinates from temp variable
let xfield = x[g=temp_total] + 0*z[g=temp_total]    

! create y (sigma/z) by summing layer thicknesses
let yfield = dp1[g=temp_total,k=@rsum]

x (longitude) is copied from my temp_total variable. To get y (depth)
I use a running sum of the layer depths. So far so good, but when I
make a plot with

shade /vlimits=250:0:50 temp_anom, xfield, yfield

the layer depths aren't interpreted correctly: 
1) the first layer is not extrapolated to 0
2) shaded "boxes" are centered on the layer depth, not drawn between two
   layer depths.

I would use bounds or edges, but I don't know how to combine this with
the 3 argument shade command, as the axis data is coming from a
variable instead of from a real axis.

Any hints on how to do this properly?

    Hein Zelle

>-----------------------------------------------------------------<
    Hein Zelle
    Dept. of Oceographic Research
    KNMI, The Netherlands
    work:        zelle@knmi.nl     http://www.knmi.nl/~zelle
    private:     hein@icce.rug.nl  http://www.icce.rug.nl/~hein
    Phone:       +31 (0)30 2206704
>-----------------------------------------------------------------<
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