Along with all the slick widget packages--BLT, ClassyTcl, and so on--which build in canvas-oriented barcharting, there's a dead-simple but too-little-known pure-Tcl technique for generating barcharts for Web applications.
[Show picture of example output [L1 ]. Show sample code. Discuss axes and legends. Explain alternatives for generation of color image. Refer to color codes.] [Cross-language comments in [L2 ].]
A different way to roughly the same visual content is by way of JavaScript. This is interesting simply for its architectural distinctiveness. [L3 ] provides examples.
"Web special effects" has related information.
It's easy enough to whip together a simple barchart on a canvas without any extension. Here's an example:
pack [canvas .c]
set y_origin 100
set x_origin 20
set heights {11 30 15 16 77 5 14 12 8 33 9}
foreach height $heights {
set x_next [expr $x_origin + 5]
.c create rectangle \
$x_origin $y_origin $x_next [expr $y_origin - $height]
set x_origin $x_next
}