Pikchr

The "behind" attribute
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The "behind object" attribute causes the object currently under construction to be drawn before the referenced object.

Pikchr normally draws objects in the order that they appear in the input script. However, the "behind" attribute can be used to alter the drawing order so that boxes used to implement background colors or borders can be drawn before the objects they enclose, even though the background-boxes are specified after the objects they enclose.

Consider this example:

Interface SQL Command Processor Virtual Machine B-Tree Pager OS Interface Tokenizer Parser Code Generator Utilities Test Code Core Backend SQL Compiler Accessories 
    lineht *= 0.4
    $margin = lineht*2.5
    scale = 0.75
    fontscale = 1.1
    charht *= 1.15
    down
IN: box "Interface" wid 150% ht 75% fill white
    arrow
CP: box same "SQL Command" "Processor"
    arrow
VM: box same "Virtual Machine"
    arrow down 1.25*$margin
BT: box same "B-Tree"
    arrow
    box same "Pager"
    arrow
OS: box same "OS Interface"
    box same with .w at 1.25*$margin east of 1st box.e "Tokenizer"
    arrow
    box same "Parser"
    arrow
CG: box same ht 200% "Code" "Generator"
UT: box same as 1st box at (Tokenizer,Pager) "Utilities"
    move lineht
TC: box same "Test Code"
    arrow from CP to 1/4<Tokenizer.sw,Tokenizer.nw> chop
    arrow from 1/3<CG.nw,CG.sw> to CP chop

    box ht (IN.n.y-VM.s.y)+$margin wid IN.wid+$margin \
       at CP fill 0xd8ecd0 behind IN
#                          ^^^^^^^^^
####################################
    line invis from 0.25*$margin east of last.sw up last.ht \
        "Core" italic aligned

    box ht (BT.n.y-OS.s.y)+$margin wid IN.wid+$margin \
       at Pager fill 0xd0ece8 behind IN
#                             ^^^^^^^^^
#######################################
    line invis from 0.25*$margin east of last.sw up last.ht \
       "Backend" italic aligned

    box ht (Tokenizer.n.y-CG.s.y)+$margin wid IN.wid+$margin \
       at 1/2<Tokenizer.n,CG.s> fill 0xe8d8d0 behind IN
#                                             ^^^^^^^^^
#######################################################
    line invis from 0.25*$margin west of last.se up last.ht \
       "SQL Compiler" italic aligned

    box ht (UT.n.y-TC.s.y)+$margin wid IN.wid+$margin \
       at 1/2<UT,TC> fill 0xe0ecc8 behind IN
#                                  ^^^^^^^^^
############################################
    line invis from 0.25*$margin west of last.se up last.ht \
      "Accessories" italic aligned

In the diagram above, the white component boxes are drawn first. Then the larger boxes that implement the various background colors are drawn relative to the component boxes. The "behind" attribute must be used to cause the background boxes to appear to be behind the component boxes. Click on the diagram to see the source text. Comments have been inserted into the source text to help identify the "behind" attributes amid all the others.