improve comment

src/frontend/plotting/plotcurv.c

@@ -130,6 +130,10 @@ ft_graf(struct dvec *v, struct dvec *xs, bool nostart)
      * interpolation.
      */
     if ((degree == 1) && (gridsize == 0)) {
+        /* We have to take care of non-monotonic x-axis values.
+        If they occur, plotting is suppressed, except for mono is set
+        to FALSE by flag 'retraceplot' in command 'plot'.
+        Then everything is plotted. */
         bool mono = (currentgraph->plottype != PLOT_RETLIN);
         dir = 0;
         for (i = 0, j = v->v_length; i < j; i++) {
@@ -137,7 +141,7 @@ ft_graf(struct dvec *v, struct dvec *xs, bool nostart)
                 realpart(xs->v_compdata[i]);
             dy = isreal(v) ? v->v_realdata[i] :
                 realpart(v->v_compdata[i]);
-            if ((i == 0 || (dir > 0 ? lx > dx : dir < 0 ? lx < dx : 0)) &&
+            if ((i == 0 || (dir > 0 ? lx > dx : (dir < 0 ? lx < dx : 0))) &&
                 (mono || (xs->v_plot && xs->v_plot->pl_scale == xs)))
             {
                 gr_point(v, dx, dy, lx, ly, 0);