Re: Gridlines on outside of clusterd columns bar chart, not in the middle
You couldn't customize directly the gridlines like you want. Options to influence the gridlines could be the type of gridlines and the scale of the axis especially the interval-steps.
Another approach would be to create another expression for the grid whereby the bars are hidden or transparent - argb(0,0,0,0) - or you used a second bar-chart which will be overlapped from the first one and you adjust there various options, like the expression, scale and other layout-settings.
Further if your chart is quite static you could consider to replace the grid with a background-image or presentation-lines from tab presentation.