struct chart_geometry;
+void chart_vector_start (cairo_t *, struct chart_geometry *,
+ const char *name);
+void chart_vector_end (cairo_t *, struct chart_geometry *);
+void chart_vector (cairo_t *, struct chart_geometry *, double x, double y);
+
/* Plot a data point */
void chart_datum(cairo_t *, const struct chart_geometry *,
int dataset UNUSED, double x, double y);