getViewPort function added

I am developing the mHealthDroid: a framework for rapid development of mHealth applications in Android.  I am working with portable health devices and I wanted to use the real time visualization but I needed to know when I have as much samples as the viewport size. I needed to know it in order to use the appendData function properly. That is, if I have received less samples that the viewport size, the appendData should not scroll to the end, otherwise appenData should scroll to the end. Thus, I added the fuction getViewPort() to make this task possible.