And here is where it all happens in axestype.pm where you can see it applying the y_number_format attrib.

sub create_y_labels
{
    my $self = shift;

    # XXX This should really be y_label_width
    $self->{y_label_len}[$_]    = 0 for 1, 2;
    $self->{y_label_height}[$_] = 0 for 1, 2;

    for my $t (0 .. $self->{y_tick_number})
    {
        # XXX Ugh, why did I ever do it this way? How bloody obscure.
        for my $axis (1 .. ($self->{two_axes} + 1))
        {
            my $label = $self->{y_min}[$axis] +
                $t * ($self->{y_max}[$axis] - $self->{y_min}[$axis]) /
                $self->{y_tick_number};
            
            $self->{y_values}[$axis][$t] = $label;

            if (defined $self->{y_number_format})
            {
                $label = ref $self->{y_number_format} eq 'CODE' ?
                    &{$self->{y_number_format}}($label) :
                    sprintf($self->{y_number_format}, $label);
            }
            
            $self->{gdta_y_axis}->set_text($label);
            my $len = $self->{gdta_y_axis}->get('width');

            $self->{y_labels}[$axis][$t] = $label;

        # TODO Allow vertical y labels
            $self->{y_label_len}[$axis] = $len 
                if $len > $self->{y_label_len}[$axis];
            $self->{y_label_height}[$axis] = $self->{yafh};
        }
    }
}
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