Always propgate any individual scale for a vector that appears

in an expression, resolving conflicts by matching length and warning
only when making an arbitary choice.  One effect of this is that it
is now possible to mix analog nodes with offset digital nodes
(an expression like dvalue+6) without a warning and get a correct plot.

src/frontend/evaluate.c

@@ -191,7 +191,7 @@ doop(char what,
     struct dvec *v1, *v2, *res;
     ngcomplex_t *c1 = NULL, *c2 = NULL, lc;
     double *d1 = NULL, *d2 = NULL, ld;
-    int length = 0, i;
+    int length = 0, i, longer;
     void *data;
     bool free1 = FALSE, free2 = FALSE, relflag = FALSE;
 
@@ -253,6 +253,7 @@ doop(char what,
     /* Make sure we have data of the same length. */
     length = ((v1->v_length > v2->v_length) ? v1->v_length : v2->v_length);
     if (v1->v_length < length) {
+        longer = 2;
         free1 = TRUE;
         if (isreal(v1)) {
             ld = 0.0;
@@ -275,6 +276,7 @@ doop(char what,
                 c1[i] = lc;
         }
     } else {
+        longer = 0;
         if (isreal(v1))
             d1 = v1->v_realdata;
         else
@@ -282,6 +284,7 @@ doop(char what,
     }
 
     if (v2->v_length < length) {
+        longer = 1;
         free2 = TRUE;
         if (isreal(v2)) {
             ld = 0.0;
@@ -335,10 +338,27 @@ doop(char what,
     }
 
     /* This is a non-obvious thing */
+
     if (v1->v_scale != v2->v_scale) {
-        fprintf(cp_err, "Warning: scales of %s and %s are different.\n",
-                v1->v_name, v2->v_name);
-        res->v_scale = NULL;
+        switch (longer) {
+        case 0:
+            if (!v1->v_scale)
+                res->v_scale = v2->v_scale;
+            else if (!v2->v_scale)
+                res->v_scale = v1->v_scale;
+            else
+                fprintf(cp_err,
+                         "Warning: scales of %s and %s are different.\n",
+                        v1->v_name, v2->v_name);
+            res->v_scale = v1->v_scale; // Do something!
+            break;
+        case 1:
+            res->v_scale = v1->v_scale;
+            break;
+        case 2:
+            res->v_scale = v2->v_scale;
+            break;
+        }
     } else {
         res->v_scale = v1->v_scale;
     }