returns the default color for the nth data series.
This may be a system wide color or a color coming from a
color scheme.
Usually there exist a fixed number of default colors. This
method should always return a valid Color. If the index (i)
is higher than the number of default colors (n), the method
should return the modulus (i mod n), i.e., the colors should
repeat in a cyclic way.
Parameter nIndex
The index of the series. This is used to obtain the
correct default color.
