This is an example of creating a derived widget - in this case for drawing a graph. The main differences are:
 - lines to be drawn are defined in 'user coordinates' not pixels
 - the coordiantes use normal graph directions (canvas pixels are defined as 0 at top, large at bottom of screen).
 - the pen colour is an option.

The code is a simplification of [Overloading widgets] and shows how to add an option to a derived widget. I do not like my use of globals here to store the widget's options.  Any improvements/suggestions sought.

 proc grapharea {w args} { ;# a graph plotter canvas derived thing.
	set dx 400
	set dy 400
	global $w.pencolour $w.xrange $w.yrange
	set $w.pencolour black
	set  $w.xrange {-1 1}
	set  $w.yrange {-1 1}
		array set options {{-colour} red}
		# split off the custom options:
		set textArgs [list]
		foreach {opt val} $args {
			switch -- $opt {
				{-pencolour} {set options($opt) $val}
				default {lappend textArgs $opt $val}
			}
		}
	eval canvas $w $textArgs

	interp hide {} $w
	# Install the alias:
	interp alias {} $w {} graphareaCmd $w
	foreach opt [array names options] {
		$w configure $opt $options($opt)
	}
  	return $w ;# the original canvas
 }
  proc graphareaCmd {self cmd args} {
	switch -- $cmd {
		configure {eval graphareaConfigure $self $cmd $args}
		cget {eval graphareaCget $self $args}
	          addline   {
			set args  [split $args] 
			set penc [$self cget pencolour];# get "local" pen colour name
			set x0 [$self cget xmin]
			set x1 [$self cget xmax]
			set y0 [$self cget ymin]
			set y1 [$self cget ymax]
			set wid [$self cget -width]
			set ht [$self cget -height]
			#puts "Xys [llength $args] $args "
			foreach {x y} $args {
		#		puts "Xys [llength $args]  $x $y"
				lappend xylist [expr $wid*double($x-$x0)/($x1-$x0)]
				lappend xylist [expr $ht*double($y-$y1)/($y0-$y1)]
			}
			interp invokehidden {} $self create line $xylist -fill $penc
		}
	}
  }
  proc graphareaConfigure {self cmd args} {
	# 3 scenarios:
	#
	# $args is empty       -> return all options with their values
	# $args is one element -> return current values
	# $args is 2+ elements -> configure the options
	switch [llength $args] {
		0 { ;# return argument values
			set result [ option get $self -pencolour ""]

			# default options:
			lappend result [interp invokehidden {} $self configure ]
			#	lappend result [uplevel 1  $self cconfigure]
			return $result
		}
		1 {
			switch -- $args {
				{-pencolour} {return [uplevel 1 [list interp invokehidden {} $self configure -pencolour]]}
				default {return [uplevel 1 interp invokehidden {} $self configure $args]}
				}
			}
		default { ;# >1 arg - an option and its value
				# go through each option:
				foreach {option value} $args {
					switch -- $option {
						{-xmin} -
						{-xmax} -
						{-ymin} -
						{-ymax} -
						{-pencolour} {
  if {1==0} {
This is the part I dont like - I create a global variable for each added option.
  }
					global $self.[string range $option 1 end]
					set $self.[string range $option 1 end] $value
					}
					default {puts " default $option, $value for $self";$self configure $option $value}
						;#$self configure $option $value
				}
			}
			return {}
		}
	}
  }
  proc graphareaCget {self args} {
	# cget defaults done by the canvas cget command
	#puts "In graphareaCget option $self $args"
	if {[info exists ::$self.$args]} {upvar #0 $self.$args val; return $val}
	switch -- $args {
		{-pencolour} {return [uplevel 1 [list interp invokehidden {} $self cget pencolour]]}
		default {return [uplevel 1 [list interp invokehidden {} $self cget $args]]}
	}
  }

	catch {destroy .fib}
 	set dr2 [grapharea .fib -width 400 -height 400 -pencolour red]
         # points are drawn with the canvas area scaled to these values:
	$dr2 configure -xmin -2 -xmax 22  -ymin -1.02 -ymax 1.02  
	pack $dr2
	$dr2 configure -pencolour blue ;# this value persist for all lines from here
	set xold 0
	set yold [expr sin(0)]
	for {set x 0} {$x<20} {set x [expr $x+.25]} {
		set y [expr sin($x)*exp(-0.1*$x)]
		$dr2 addline $xold $yold $x $y
		set xold $x
		set yold $y
	}
	$dr2 configure -pencolour orange
	set xold 0
	set yold [expr cos(0)]
	for {set x 0} {$x<20} {set x [expr $x+.25]} {
		set y [expr cos($x)*exp(-0.1*$x)]
		$dr2 addline $xold $yold $x $y
		set xold $x
		set yold $y
	}
	$dr2 configure -pencolour green
	# a simple rectangle drawn as a line with 5 coordinates (10 values, last repeats first)
	$dr2 addline  0 1 20 1 20 -1 0 -1 0 1
	# you can also add canvas options to the grapharea:
	interp invokehidden {} $dr2  create rectangle 20  20 390 390
	interp invokehidden {} $dr2  create text 120 30  -fill black -text "Graph Area"