add multiplot layout options: scale, offset, margins, spacing ; depre…

…cate old style of speciyfing them.

previously scale & offset were provided as separate multiplot options,
which were rendered as plain numbers independent of the layout option.
gnuplot 5.4 doesn't recognize that syntax. they must be part of the
layout option.

Now, they are specified inline in the arrayref passed to the layout option.

lib/PDL/Graphics/Gnuplot.pm

@@ -4235,19 +4235,74 @@ The options hash will accept:
 
 =item layout - define a regular grid of plots to multiplot
 
-C<layout> should be followed by an ARRAY ref that contains at least
-number of columns ("NX") followed by number of rows ("NY).  After
-that, you may include any of the "rowsfirst", "columnsfirst",
-"downwards", or "upwards" keywords to specify traversal order through
-the grid.  Only the first letter is examined, so (e.g.) "down" or even
-"dog" works the same as "downwards".
+C<layout> should be followed by an ARRAY ref with the following
+required and optional elements:
+
+=over
+
+=item NX
+
+I<required>
+
+The integer number of columns.
+
+=item NY
+
+I<required>
+
+The integer number of rows.
+
+=item C<rowsfirst> | C<columnsfirst>
+
+=item C<downwards> | C<upwards>
+
+I<Optional>
+
+These take no arguments.
+
+Traversal order through the grid.  Only the first letter is examined,
+so (e.g.) "down" or even "dog" works the same as "downwards".
+
+=item C<scale> => SCALAR | ARRAYREF
+
+I<Optional>
+
+A scale to be applied to each plot.  If a scalar is specified, that
+will be applied to both X and Y; if an ARRAY ref is specified,
+it should contain independent values for each axis.
+
+=item C<offset> => SCALAR | ARRAYREF
+
+I<Optional>
+
+An offset to be applied to each plot.  If a scalar is specified, that
+will be applied to both X and Y; if an ARRAY ref is specified,
+it should contain independent values for each axis.
+
+=item C<spacing> => SCALAR | ARRAYREF
+
+I<Optional>
+
+Gap sizes between two adjacent plots. to each plot.  If a scalar is
+specified, that will be applied to both X and Y; if an ARRAY ref is
+specified, it should contain independent values for each axis.
+
+=item C<margins> => ARRAYREF
+
+I<Optional>
+
+Accepts an ARRAY ref with four values ( [ I<< <left>,<right>,<bottom>,<top> >> ] ).
+
+=back
 
 =item title - define a title for the entire page
 
 C<title> should be followed by a single scalar containing the title string.
 
 =item scale - make gridded plots larger or smaller than their allocated space
 
+[Deprecated; add this to the layout keyword]
+
 C<scale> takes either a scalar or a list ref containing one or two
 values.  If only one value is supplied, it is a general scale factor
 of each plot in the grid.  If two values are supplied, the first is an
@@ -4256,6 +4311,8 @@ stretch factor for each plot in the grid.
 
 =item offset - offset each plot from its grid origin
 
+[Deprecated; add this to the layout keyword]
+
 C<offset> takes a list ref containing two values, that control placement
 of each plot within the grid.
 
@@ -4284,23 +4341,37 @@ Ends a multiplot block (i.e. a block of plots that are meant to render to a sing
 our $mpOptionsTable = {
     'layout' => [sub { my($old, $new, $h) = @_;
 		       my ($nx,$ny);
-		       my @dirs=("","");
+                       my @output;
+		       my %dirs;
 		       if(!ref($new)) {
 			   $nx = $ny = $new;
 		       } elsif(ref($new) eq 'ARRAY') {
 			   $new = [1] if(@$new == 0);
 			   $nx = shift @$new;
 			   $ny = (@$new) ? shift @$new : $nx;
 			   while($_ = shift @$new) { # assignment
-			       $dirs[0]="rowsfirst"    if(m/^r/i);
-			       $dirs[0]="columnsfirst" if(m/^c/i);
-			       $dirs[1]="downwards"    if(m/^d/i);
-			       $dirs[1]="upwards"      if(m/^u/i);
+
+                               if ( my ($command) = /^(scale|offset|spacing|margins)$/ ) {
+                                   barf "multiplot: missing option to $command"
+                                     unless @$new;
+                                   $_ = shift @$new;
+                                   my @values = ref($_) eq 'ARRAY' ? @{$_} : $_;
+                                   barf ( "multiplot: $command requires four values" )
+                                     if $command eq 'margins' && @values != 4;
+                                   barf ( "multiplot: $command requires 1 or 2 values" )
+                                     if $command ne 'margins' && ( @values != 1 or @values != 2 );
+                                   push @output, $command, join( q{,}, @values );
+                               }
+			       $dirs{0}="rowsfirst"    if(m/^r/i);
+			       $dirs{0}="columnsfirst" if(m/^c/i);
+
+			       $dirs{1}="downwards"    if(m/^d/i);
+			       $dirs{1}="upwards"      if(m/^u/i);
 			   }
 		       } else {
 			   barf "multiplot: layout option needs a scalar or array ref value\n";
 		       }
-		       return join(" ",("$ny,$nx",$dirs[0],$dirs[1]));
+		       return join(" ", "$ny,$nx", @output, values %dirs);
 		 },
 		 'cl',undef,1,''],
     'title' => ['s','cq',undef,2,''],