A little bar chart

Summary edit

Richard Suchenwirth 2005-02-27: Looking at bar charts in magazines, I had the usual idea: I wanted to have them in Tcl too, and on the iPaq.

AM As of version 1.3, this is also part of Tklib's Plotchart

Description edit

So in this weekend fun project, I started with pseudo-3-dimensional bars - a rectangle in front as specified, embellished with two polygons - one for the top, one for the side:

proc 3drect {w args} {
    if [string is int -strict [lindex $args 1]] {
        set coords [lrange $args 0 3]
    } else {
        set coords [lindex $args 0]
    }
    foreach {x0 y0 x1 y1} $coords break
    set d [expr {($x1-$x0)/3}]
    set x2 [expr {$x0+$d+1}]
    set x3 [expr {$x1+$d}]
    set y2 [expr {$y0-$d+1}]
    set y3 [expr {$y1-$d-1}]
    set id [eval [list $w create rect] $args]
    set fill [$w itemcget $id -fill]
    set tag [$w gettags $id]
    $w create poly $x0 $y0 $x2 $y2 $x3 $y2 $x1 $y0 -fill [dim $fill 0.8] -outline black
    $w create poly $x1 $y1 $x3 $y3 $x3 $y2 $x1 $y0 -fill [dim $fill 0.6] -outline black -tag $tag
}

For a more plastic look, the fill color of the polygons is reduced in brightness ("dimmed"):

proc dim {color factor} {
   foreach i {r g b} n [winfo rgb . $color] d [winfo rgb . white] {
      set $i [expr int(255.*$n/$d*$factor)]
   }
   format #%02x%02x%02x $r $g $b
}

Draw a simple scale for the y axis, and return the scaling factor:

proc yscale {w x0 y0 y1 min max} {
   set dy   [expr {$y1-$y0}]
   regexp {([1-9]+)} $max -> prefix
   set stepy [expr {1.*$dy/$prefix}]
   set step [expr {$max/$prefix}]
   set y $y0
   set label $max
   while {$label>=$min} {
       $w create text $x0 $y -text $label -anchor w
       set y [expr {$y+$stepy}]
       set label [expr {$label-$step}]
   }
   expr {$dy/double($max)}
}

An interesting sub-challenge was to round numbers very roughly, to 1 or maximally 2 significant digits - by default rounding up, add "-" to round down:

proc roughly {n {sgn +}} {
   regexp {(.+)e([+-])0*(.+)} [format %e $n] -> mant sign exp
   set exp [expr $sign$exp]
   if {abs($mant)<1.5} {
       set mant [expr $mant*10]
       incr exp -1
   }
   set t [expr round($mant $sgn 0.49)*pow(10,$exp)]
   expr {$exp>=0? int($t): $t}
}

So here is my little bar chart generator. Given a canvas pathname, a bounding rectangle, and the data to display (a list of {name value color} triples), it figures out the geometry. A gray "ground plane" is drawn first. Note how negative values are tagged with "d"(eficit), so they look like they "drop through the plane".

proc bars {w x0 y0 x1 y1 data} {
    set vals 0
    foreach bar $data {
        lappend vals [lindex $bar 1]
    }
    set top [roughly [max $vals]]
    set bot [roughly [min $vals] -]
    set f [yscale $w $x0 $y0 $y1 $bot $top]
    set x [expr $x0+30]
    set dx [expr ($x1-$x0-$x)/[llength $data]]
    set y3 [expr $y1-20]
    set y4 [expr $y1+10]
    $w create poly $x0 $y4 [expr $x0+30] $y3  $x1 $y3 [expr $x1-20] $y4 -fill gray65
    set dxw [expr $dx*6/10]
    foreach bar $data {
        foreach {txt val col} $bar break
        set y [expr {round($y1-($val*$f))}]
        set y1a $y1
        if {$y>$y1a} {swap y y1a}
        set tag [expr {$val<0? "d": ""}]
        3drect $w $x $y [expr $x+$dxw] $y1a -fill $col -tag $tag
        $w create text [expr {$x+12}] [expr {$y-12}] -text $val
        $w create text [expr {$x+12}] [expr {$y1a+2}] -text $txt -anchor n
        incr x $dx
    }
    $w lower d
}

#-- Generally useful helper functions:

proc max list {
    set res [lindex $list 0]
    foreach e [lrange $list 1 end] {
        if {$e>$res} {set res $e}
    }
    set res
}
proc min list {
    set res [lindex $list 0]
    foreach e [lrange $list 1 end] {
        if {$e<$res} {set res $e}
    }
    set res
}
proc swap {_a _b} {
    upvar 1 $_a a $_b b
    foreach {a b} [list $b $a] break
}

Testing the whole thing:

pack [canvas .c -width 240 -height 280]
bars .c 10 20 240 230 {
    {red 765 red}
    {green 234 green}
    {blue 345 blue}
    {yel-\nlow 321 yellow}
    {ma-\ngenta 567 magenta}
    {cyan -123 cyan}
    {white 400 white}
}
.c create text 120 10 -anchor nw -font {Helvetica 18} -text "Bar Chart\nDemo"
#-- iPaq specialties - not needed in general
wm geometry . +0+0
bind . <Up> {exec wish $argv0 &; exit}

SS: very nice, I wish there was a "porting" simple to use with CGI applications, i.e. generating an image.

JM 2005-06-21: If you feed the chart just with numbers less than 1. you will get an error caused by the incr being used with non-integer values.

instead of:

incr label -$step

use:

set label [expr $label - $step]

in the proc "yscale".

RS: Good hint - thanks, fixed above.

Category Graphics

Category Plotting

Category Visualization

Arts and Crafts of Tcl-Tk Programming

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