BL0X3R was on the money, but I will add graph it. When I graph our encoder data, depending on a number of factors I have seen very "jittery" velocity data (solutions are as mentioned, calculate yourself, if necessary calculate an average velocity over the last n encoder reads).
If you are using labVIEW visualizing data in real time is trivial. If not, I have taken to writing my logs in JSON and then using javascript and the browser for visualizing (I use
http://www.flotcharts.org/), rather than trying to write code in C++ or Java to display graphs.
In many cases seeing is understanding.