improve a spline

When you run the following code, a canvas comes up and you can click-hold your mouse and drag it around to create bezier curves.
The line is jagged and it has a double curve. I wonder if it is possible to make it smooth and reduce it to a plain s shape. Any ideas?
package req Tk
canvas .c -width 500 -height 500
pack .c -fill both -expand 1
bind .c <Button-1>  [list mark %x %y]
bind .c <B1-Motion> [list draw %x %y]
proc mark {x y} {
  set ::id 0
  set ::ax $x
  set ::ay $y
}
proc draw {x y} {
  .c delete $::id
  set xdiff [expr {($x - $::ax) / 2.0}]
set tx2 [expr {$::ax + 2 * $xdiff}]
  set tx3 [expr {$x   - 2 * $xdiff}]
  set ::id [.c create line $::ax $::ay $tx2 $::ay $tx3 $y $x $y \
         -fill #494949 -width 3 -smooth bezier -splinesteps 24]
}