To affect the numeric labels, you just need to scale that axis' numbers to suit your new units. To set the title, use the env method. From the PDL::Graphics::PGPLOT::Window pod:

$rate_win = PDL::Graphics::PGPLOT::Window->new(Device => '/xw',
            Aspect => 1, WindowWidth => 5, NXPanel => 2);

$area_win = PDL::Graphics::PGPLOT::Window->new(Device => '/xw',
            Aspect => 1, WindowWidth => 5);

$rate_win->env(0, 10, 0, 1000, {XTitle => 'Days', YTitle => '#Rabbits'
+});
$rate_win->env(0, 10, 0, 100, {Xtitle=>'Days', Ytitle => 'Rabbits/day'
+});

$area_win->env(0, 1, 0, 1, {XTitle => 'Km', Ytitle => 'Km'});
# And theoretical prediction.
$rate_win->line(sequence(10), fibonacci(10), {Panel => [1, 1]});
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After Compline,
Zaxo