New property for the DownSampling function.

The DownSampling function has a new method, MinMaxFirstLastNull, which for every group of points plots the first, maximum, minimum and last point within it and also includes the first null point of the group in the calculation but without plotting it. The DisplayedPointCount specifies how many points the DownSampling function should paint and displays this number as a maximum number of points. Try zooming on the chart below to see the effect.