The following function

$$f(x)=-2 \cos \left(\sqrt{2} \arccos \left(\frac{x-1}{2}\right)\right)+1$$

has interesting property to form a continuous curve with its own integer iterations. The following image illustrates this property:

alt text http://static.itmages.ru/i/10/1201/h_1291177404_9062e5977d.png

Here blue is f(x), red is f(f(x)), yellow is f(f(f(x))) and green is f(f(f(f(x)))). It seems that all these functions form a continuous, and, probably, smooth curve.

The question is what is the general criterion for a function to have such property. Can you point some more examples of functions with such property?