Gridlines between values

Currently gridlines will only appear on the category values. I would like the option to have the gridelines go between values. This would be especially useful on bar charts when you have multiple series per category value. The line would help visually separate the grouping of series by the categorical value. Right now the line is only in the middle and it makes the visual very difficult to see where one group ends and the other starts.

If you also do a trellis this the visual clutter is compounded, whereas having the gridline between category values would really help the reader understand the relationship of the trellis aligned by the categorical value acorss the trellis.