Custom bar data gaps 459

As eichsjul reported the documentation of of gaps in the data for customBars and errorBars doesn't fit the actual implementation. (see "dygraphs Data Format" in http://dygraphs.com/data.html#array)
If setting the whole sample array to null as described in the documentation an exception is thrown.
This bugfix, fixes this problem so that both arrays with null values as well as a null value instead of the array are supported by examining the data and normalizing the null values to fit the specified graph type.