#123 -leftvariable is constantly touched

[Bryan Oakley]

blt 2.4z running on MacOSX/X11, tcl/tk 8.4.9

When using -leftvariable and friends I find that the
variable is continuously touched when data is added to
the graph, even if the margins don't change. This seems
excessive.

The following code will print once a second "margin
changed <n>", where <n> never changes after a couple
iterations

package require BLT
proc marginChanged {args} {
global margin
puts "margin changed: $margin(left)"
}

proc generateData {} {
x append [clock seconds]
y append [expr {rand()}]
after 1000 generateData
}

blt::vector create x y
blt::stripchart .s -leftvariable margin(left)
.s element create line1 -xdata x -ydata y
trace add variable margin write marginChanged

generateData
pack .s