code fragments for rubberband selection

proc startSelection {x y} {
    variable xstart
    variable ystart
    set xstart [.c canvasx $x]
    set ystart [.c canvasy $y]
    .c create rectangle $xstart $ystart $xstart $ystart -tags selector \
	-outline green -dash - -fill {}
}

proc adjustSelection {x y} {
    variable xstart
    variable ystart
    set x [.c canvasx $x]
    set y [.c canvasy $y]
    .c coords selector $xstart $ystart $x $y
    return [list $xstart $ystart $x $y]
}

proc endSelection {x y b} {
    lassign [adjustSelection $x $y] x0 y0 x1 y1
    .c delete selector
    # At this point you probably want to do [.c find overlapping $x0 $y0 $x1 $y1] and
    # do something with the result.
}

bind .c <ButtonPress-1> [list startSelection %x %y]
bind .c <B1-Motion> [list adjustSelection %x %y]
bind .c <ButtonRelease-1> [list endSelection %x %y %b]