Listing 5. Stage 4 in Tcl/Tk, out.tcl

canvas .p -width  210  -height  210

.p create line      5 105  205  105

.p create line    105   5  105  205

.p create text    180  90 -text "x"

.p create text    120  10 -text "x'"

pack .p

proc draw { } {

  if { ([gets stdin line] >= 0) && \

       ([scan $line "%f %f" x y] >= 2) } {

    set x [expr    25 * $x + 105]

    set y [expr -12.5 * $y + 105]

    .p create oval $x $y $x $y

    after 1 { draw }

  }

}

after 0 { draw }