+ A chart is abstract. Every actual chart is a subclass of chart. */
+
+#include <stdbool.h>
+
+/* A chart.
+
+ The members of struct chart should not be accessed directly. Use one
+ of the accessor functions defined below. */
+struct chart
+ {
+ int ref_cnt;
+ const struct chart_class *class; /* Subclass. */
+ char *title; /* May be null if there is no title. */
+ };