+ * Action on starting or continuing a mouse drag. There are two possible
+ * actions:
+ * <ul>
+ * <li>drag up or down on a graphed annotation increases or decreases the
+ * height of the graph</li>
+ * <li>dragging left or right selects the columns dragged across</li>
+ * </ul>
+ * A drag on a graph annotation is treated as column selection if it starts
+ * with more horizontal than vertical movement, and as resize if it starts
+ * with more vertical than horizontal movement. Once started, the drag does
+ * not change mode.