When the fill color property of `DataSeries.plotLabel` using a
DataSeriesValues on a line chart is set, the XLSX file written
is corrupted, and MSExcel2016 removes the drawing1.xml if forced open.
This problem was already documented on issue #589 along with a possible
solution. So all credits go to @madrussa. I am only submitting the PR.
Fixes#589Closes#1930
Tiago Malheiro