Index: scripts/general/interp1.m
===================================================================
RCS file: /cvs/octave/scripts/general/interp1.m,v
retrieving revision 1.4
diff -u -r1.4 interp1.m
--- scripts/general/interp1.m	1 Mar 2007 15:57:50 -0000	1.4
+++ scripts/general/interp1.m	1 Jun 2007 06:59:02 -0000
@@ -196,6 +196,7 @@
   elseif (strcmp (method, "linear"))
     dy = y(2:ny,:) - y(1:ny-1,:);
     dx = x(2:nx) - x(1:nx-1);
+    if (!(all(dx(:)>0) || all(dx(:)<0))) error("interp1: X must be strictly monotonic"); endif
     if (pp)
       yi = mkpp (x, [dy./dx, y(1:end-1)], szy(2:end));
     else