Cooperativity of haemoglobin and oxygen dissociation curve of haemoglobin
As you may notice from the graph you provided, there actually is no such thing as true 100% saturation: there is always an asymptotic approach to 100%, regardless of cooperativity.

Let's think, though, of what is actually plotted in that graph: $p_{O_2}$ effectively oxygen concentration, versus number of total percentage of binding sites that have an $O_2$ bound.

However, this also means that as you go up on the y-axis, the number of sites available to bind also goes down. So even if those sites have a high affinity for oxygen, there aren't as many of them open!

That's one big problem with the type of plot you show: it doesn't highlight the cooperativity very well. Therefore, people sometimes plot this information in a "Hill plot", with an algebraic shuffling and log scaling like this:



See Wikipedia

...where $\theta$ represents the fraction of receptors bound to ligand, and $L$ is the (unbound) ligand concentration. $n$ is the Hill Coefficient, i.e., the cooperativity.

When plotted this way, you can easily see the effect of the cooperativity of hemoglobin versus myoglobin (which has cooperativity of 1, because it has a single binding site):



http://cbc.arizona.edu/classes/bioc462/462a/NOTES/hemoglobin/hemoglobin_function.htm via Fig. 7-13, Nelson &amp; Cox Principles of Biochemistry, 3rd ed., 2000