I decided that using 4 SPACES is better than mixing with tabs.

box.h

@@ -24,22 +24,22 @@ struct box {
     double fills;
 
     union {
-    	/* button */
-	    char *name;
+        /* button */
+        char *name;
 
-    	/* chart and vertical/horizontal box */
-    	struct {
-    	    /* chart and vertical/horizontal box */
-    	    size_t length;
+        /* chart and vertical/horizontal box */
+        struct {
+            /* chart and vertical/horizontal box */
+            size_t length;
 
-    	    union {
-        	    /* vertical/horizontal box */
-        	    struct box **children;
+            union {
+                /* vertical/horizontal box */
+                struct box **children;
 
-            	/* chart */
-        	    double *prices;
-        	};
-    	};
+                /* chart */
+                double *prices;
+            };
+        };
     };
 };
 
@@ -49,5 +49,6 @@ struct box *new_vertical_box(void);
 struct box *new_horizontal_box(void);
 struct box *new_chart_box(void);
 
+void append_box(struct box *parent, struct box *child);
 
 void draw_box(struct box *, double x, double y, double width, double height);