GeoGebra announced the release of their GeoGebra Graphing Calculator earlier this month; currently available for Android, the app will also be available for iPhone and Windows – watch for announcements.
Having downloaded the app to my phone I can confirm it works really well and i was easily able to follow the instructions given in the links here.
I very quickly created the diagram in the screenshot from my phone below. Using a finger one can drag point D around and note the angle at the circumference. With an account you can also save your files which are standard GeoGebra files.