Gantt Chart Crashes With backgroundColor:{fill:'transparent'}

[Chris G]

fill:'transparent' works properly with most chart types, but for a Gantt Chart it causes the red chart error 'none' is not a valid hex color. In my tests, it's the only chart type that has this problem. 

I don't need a fix, I have implemented a workaround. But it would be nice to fix it, since it took me some time to identify the problem.