Smart update of the graphics window

Normally a display() starts with an implicit cls(). This is perfect as long as it doesn't happen too often. During measurements, an update of the screen is more appropriate than a complete re-display. That is the purpose of the "update" => "yes" switch, see the following example:

use Spectra;

delete();
cls();

my $np = 401;

my $sin = SCAN->create( name => "sinus", 
			start => 0,
			stop => 10,
			title => "Sinus",
			np => "$np",
			at => "(1,2,1)");
my $tan = SCAN->create( name => "tangens", 
			start => 0,
			stop => 10,
			title => "Tangens",
			np => $np,
			at => "(1,2,2)");

$sin->calc( "expr" => "sin(x(sinus))");
$tan->calc( "expr" => "tan(x(tangens))");

foreach my $i( 1 .. $np)
{
    $sin->set( curr => $i);
    $tan->set( curr => $i);
    Autoscale( y => "yes");
    display( update => "yes");
}