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my @deltas = map{ ( 0 <=> $y[$_-1] - $y[$_] ) } 1 .. $#y;

-1 1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 1 1 1 1 1 1 1 1 -1 -1 -1 -1 1 1 
+1 1 1 1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 1 1 1 1 1 1 1

$deltas[$_-1] != $deltas[$_] and $im->line( $x[$_], 0, $x[$_], 800, 0 
+) for 1 .. $#x-1;

#! perl -slw
use strict;
...
close PNG;

system 1, "$0.png";

__DATA__
4.1330 4580.87
...
4.7470 1664.15     28832.15      506969.93
4.7600 2098.54     35555.08
4.7730 2588.58