y=x^2
or
y=x**2
or
y=3x^2


##</code><code>##

#!/usr/bin/perl -wT
use strict;

use constant pi => 3.14159265358979;

use Tk;
my $top = new MainWindow(-width => 800, -height => 600);
my $cvs = $top->Canvas(-width => 800, -height => 550);
my $frm = $top->Canvas(-width => 800, -height => 20);
my $txt = $frm->Entry(-width => 40);
my $min = $frm->Entry(-width => 5);
my $max = $frm->Entry(-width => 5);
my $stp = $frm->Entry(-width => 5);
my $clr = $frm->Button(-text => 'Clear');
$clr->bind('<Button-1>' => [ sub { $cvs->delete($_) for $cvs->find('all') } ]);
$txt->bind('<KeyPress-Return>', [ \&evaluate ]);
$frm->pack(-side => 'bottom');
$min->pack(-side => 'left');
$max->pack(-side => 'left');
$stp->pack(-side => 'left');
$clr->pack(-side => 'right');
$txt->pack(-side => 'right');
$cvs->pack(-side => 'top');

$min->insert(0, "0");
$max->insert(0, "100");
$stp->insert(0, "1");
        
MainLoop;

sub evaluate
{
  my $text = $txt->get();
  my $func = lc $text;
  $func =~ s/(\d+)(pi|x)/$1 * $2/g;
  $func =~ s/([xy])/\$$1/g;
  $func =~ s/\^/**/g;
  my $minx = $min->get();
  my $maxx = $max->get();
  my $step = $stp->get();
  my $size =   1;
  for (my $x = $minx; $x <= $maxx; $x += $step) {
    my $y;
    eval "$func";
    # warn "$func: $x, $y\n" if !($x % 50);
   $cvs->createRectangle(
      800 * $x/$maxx,         300 - $y,
      800 * $x/$maxx + $size, 300 - $y + $size,
      -width => 1);
  }
  $txt->delete(0, length $text);
}
__END__