ttyplot.c: Fix clock animation

Regression from merge commit 95467bf:
> # git diff 4901969 95467bf | tail -n6
>
>  redraw_and_continue:
> -        time(&t1);  // to animate the clock display
>          redraw_screen(errstr);
>      }
>

ttyplot.c

@@ -638,6 +638,7 @@ int main(int argc, char *argv[]) {
             td=derivative(&values1[n], two ? &values2[n] : NULL, &now);
 
 redraw_and_continue:
+        gettimeofday(&now, NULL);
         redraw_screen(errstr);
     }