Gestures and usability

I'm currently drawing some mockups of my future iPhone app.

One of the app's functionalities is to display a bar graph showing the evolution of a value over time. Users can perform few gestures on the graph :

• swipe/drag to move through time;
• pinch to zoom in or zoom out (and therefore display a longer or shorter period of time);
• double tap to add a cursor to the graph (i.e. a vertical line with a label on top).

What I'm afraid of is users not noticing these gestures. Of course, I would p开发者_开发百科rovide buttons for doing the same tasks, but if users ended up only using those, the interface's usability would not be very great...

Therefore, I am wondering if there is any way to show some visual clues to indicate the presence of gestures on the interface. Do you know any app that does something similar?

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I think if you animate mentioned graph behavior it would be a great clue for user to perform this actions by fingers. For example if he(she) choose another date you should move you graph through time smoothly with easyInOut animation. Or if user changed scale you should gradually zoom from scale 1 to scale 2.