When a big slog2 file like the one shown in Figure 3.10is viewed, the whole timeline canvas is filled with preview drawables. Although it provides a reasonable description at a high level,3.4 it is hard to know the details. Hence, a well-designed zoomable and scrollable user interface (ZSUI) of the timeline canvas becomes necessary to help the viewer locate events of interest. The ZSUI of the timeline canvas includes many parts and operations. The most handy ones are dragged zoom, grasp and scroll and instant zoom in and out. All these features are supported by the zoomable and scrollable canvas. There are two such canvases in the Timeline window: timeline canvas and time ruler canvas. In these canvases, left mouse clicking can be alternated in two different modes by a pair of toggled buttons as shown in Figures 3.11 and 3.12. They are called zoom and hand modes. Each canvas in the Timeline window has its own set of toggled buttons that determine its left mouse click behavior. The timeline canvas's toggled buttons are located above the canvas at the end of the time display panel. The time ruler's toggled buttons are located at the bottom of row adjustment panel, next to the end of the ruler. By default, the timeline canvas is in zoom mode, and the time ruler canvas is in hand mode, so the user can do zooming when the cursor is in the timeline canvas and can scroll easily by simply moving the cursor over the ruler canvas. Also, the scrolling can be done by simply dragging on the scrollbar's knob, clicking the end buttons and in the space between the knob and scrollbar's end buttons.