I explicitly ignore all lines that have '-' values

These may come from vnl-filter -p "dx=diff(x)" ... output. For instance, this
plot is broken before this patch:

  (echo '# x'; seq 5) | vnl-filter --noskipempty -p 'd=diff(x)' \
  | feedgnuplot --lines

Changes

@@ -1,3 +1,14 @@
+feedgnuplot (1.62)
+
+  * I explicitly ignore all lines that have '-' values
+    These may come from vnl-filter -p "dx=diff(x)" ... output. For instance, this
+    plot is broken before this patch:
+
+      (echo '# x'; seq 5) | vnl-filter --noskipempty -p 'd=diff(x)' \
+      | feedgnuplot --lines
+
+ -- Dima Kogan <dkogan@debian.org>  Sat, 27 Jan 2024 17:01:18 -0800
+
 feedgnuplot (1.61)
 
   * #! line uses /usr/bin/env to work with nonstandard installations

bin/feedgnuplot

@@ -16,7 +16,7 @@ use Pod::Usage;
 use Time::Piece;
 
 # Makefile.PL assumes this is in ''
-my $VERSION = '1.61';
+my $VERSION = '1.62';
 
 my %options;
 interpretCommandline();
@@ -780,6 +780,12 @@ sub mainThread
       $options{curvestyleall} = "with circles $options{curvestyleall}";
     }
 
+    # Required to ignore all lines that have '-' values, such as may come from
+    # vnl-filter -p "dx=diff(x)" ... output. For instance, this plot is broken
+    # without this extra line:
+    #   (echo '# x'; seq 5)  | vnl-filter --noskipempty -p 'd=diff(x)' | ./feedgnuplot --lines
+    print PIPE "set datafile missing \"-\"\n";
+
     print PIPE "set style data $style\n" if $style;
     print PIPE "set grid\n";