Allow to draw gridlines on top of data

[jiho]

When using geom_tile or maps, the grid lines are completely obscured by the data plotted on top of them. Sometimes, particularly for maps, it is desirable to still see the grid lines. Using transparency is a workaround but costly in terms of rendering, compatibility (EPS does not support transparency) and legibility.

Ideally, one should be able to specify where to draw grid lines (at which level in the plot). For example, one might want to draw them on top of a map/tile layer but below a points layer.

Another way to look at it is to specify, for each geom/layer wether is it above or below the grid lines.

[hadley]

This sounds like a great feature, but unfortunately we don't currently have the development bandwidth to support it. If you'd like to submit a pull request that implements this feature, please follow the instructions in the development vignette.