''From a news:comp.lang.tcl posting by [Roger E Critchlow] on Aug 17, 1999'' -- 

Create a window, $w, with an analog dial of $diameter pixels, 
background color $bg, foreground color $fg, and middleground
color $mg.  The dial will update the frequency stored in the
global named $var between minimum value $min and maximum value
$max.  Grab the dial anywhere with the left mouse button and
spin, the marker that spins is only there to give you some
rotational feedback.  The concentric regions marked on the dial
give different update speeds depending on which region you grab
with the mouse.  The mouse is tracked anywhere on the screen as
long as the button remains depressed, so you can get nice fine
adjustments by pulling the mouse way outside the dial.

======
 proc dial {w var min max diameter bg fg mg} {
    # create the dial canvas
    canvas $w -width $diameter -height $diameter -bg $bg \
        -highlightthickness 0
    # create a data array
    upvar \#0 $w data
    # cleanup the data array
    bind $w <Destroy> [list unset $w]
    # save arguments
    foreach v {w var min max diameter bg fg mg} {
        set data($v) [set $v]
    }
    # compute some values
    set data(x0) [expr {$diameter/2}]
    set data(y0) [expr {$diameter/2}]
    set data(r0) [expr {($diameter-10)/2}]
    set data(dr) [expr {$data(r0)/5}]
    set data(mark-theta) 250
    set data(2pi) [expr {2*atan2(0,-1)}]
    # create the dial disk
    $w create oval 5 5 [expr {$diameter-5}] [expr {$diameter-5}] \
        -fill $fg -outline $mg -width 4 \
        -tags dial
    # mark the resolution radii with concentric circles
    foreach d {2 3 4} {
        set r [expr {$d*$data(dr)}]
        $w create oval \
            [expr {$data(x0)-$r}] [expr {$data(y0)-$r}] \
            [expr {$data(x0)+$r}] [expr {$data(y0)+$r}] \
            -fill {} -outline $mg -width 4 \
            -tags dial
    }
    # draw a rotating marker
    $w create line $data(x0) $data(y0) $data(x0) 5 \
        -width 4 -fill $mg -arrow last \
        -tags {dial mark}
    # press binding
    $w bind dial <ButtonPress-1> {dial-press %W %x %y}
    # motion binding
    $w bind dial <B1-Motion> {dial-motion %W %x %y}
    # return our window
    return $w
 }
 proc dial-press {w x y} {
    # bind to data array
    upvar \#0 $w data
    # determine radius and theta
    set x [expr {$x-$data(x0)}]
    set y [expr {$data(y0)-$y}]
    set data(r) [expr {sqrt($x*$x+$y*$y)}]
    set data(theta) [expr {int(1000*atan2($y,$x)/$data(2pi))}]
    # compute frequency update for this radius
    # this assumes the 1000 counts/rev done below
    switch [expr {int(($data(r0)-$data(r))/$data(dr))}] {
        0 {
            set data(kHz/count) 0.01;   # 10 kHz/rev
        }
        1 {
            set data(kHz/count) 0.1;    # 100 kHz/rev
        }
        2 {
            set data(kHz/count) 1;      # 1 MHz/rev
        }
        default {
            set data(kHz/count) 10;     # 10 MHz/rev
        }
    }
 }
 proc dial-motion {w x y} {
    # bind to data array
    upvar \#0 $w data
    # compute dtheta
    set x [expr {$x-$data(x0)}]
    set y [expr {$data(y0)-$y}]
    set dtheta [expr {(int(1000*atan2($y,$x)/$data(2pi))-$data(theta))%1000}]
    if {$dtheta > 500} { set dtheta [expr {$dtheta-1000}] }
    # update theta
    set data(theta) [expr {($data(theta)+$dtheta)%1000}]
    # update marker
    set data(mark-theta) [expr {$data(mark-theta)+$dtheta}]
    set t [expr {$data(2pi)*$data(mark-theta)/1000}]
    $w coords mark $data(x0) $data(y0) \
        [expr {$data(r0)*cos($t)+$data(x0)}] \
        [expr {$data(y0)-$data(r0)*sin($t)}]
    # update frequency
    upvar \#0 $data(var) frequency
    set f [expr {$frequency-$dtheta*$data(kHz/count)}]
    if {$f < $data(min)} {
        set f $data(min)
    } elseif {$f > $data(max)} {
        set f $data(max)
    }
    # update frequency variable
    set frequency [format %12.2f $f]
 }

 # Here's some demo code:
 set bg \#4f4f4f
 set fg grey
 set mg white
 set frequency 15000.00
 . configure -bg $bg
 pack [frame .f -bg $bg] -side top -fill x
 pack [label .f.l -font {-size 32} -width 9 -anchor e \
        -textvar frequency -bg $bg -fg $mg] -side left
 pack [label .f.hz -font {-size 32} -text "kHz" -bg $bg -fg $mg] -side  left
 pack [dial .d frequency 0.00 30000.0 200 $bg $fg $mg] -side top
======

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