It is possible to set the properties (colour, etc) of the border and of the fill for each of the bars
One property is to define its transparency
If you make the border transparent, what appears is not what is underneath the bar (eg gridlines, chart background, possibly, in the case of bars plotted on the secondary axis, the bars plotted on the primary axis etc) but what is underneath the border namely the fill. This is less than helpful!
Of course, it may well be that I am doing something wrong in which case appropriate advice would be most welcome.
Whilst on the properties of bars in charts: given that the bars are dynamic in their width, it is curious that the thickness of the bar is specified in an absolute measure (pts) as opposed to something more relative like %