added support for missing values and added buffered drawing (great performance improvement)

Added support for missing values (if Y value is NaN it is not drawn, datapoint between two NaN values are drawn as a dot)

Added buffered drawing (great performance improvement!)

ToDo: move the creation of the bitmap/canvas away from the OnDraw event (should only be done at initialisation or orientation change)
ToDo: manage scaling when only NaN datapoints are in the series. (skip scaling completely perhaps, or scale to a default range)