Are strips and panel grids 'guides'?

[teunbrand]

I'm aware that they currently aren't part of any user-facing guide system. This issue is to explore thought on whether they should be (after guides are ported to ggproto).

Arguably, one might see the panel grid as a guide for the panel. There is even a guide_grid() function that coord_cartesian() and coord_trans() uses to draw the panel grid. I think it might be useful to make the panel grid a formal, extensible guide. Here are some examples for which that might be useful:

• Draw major grid lines in between breaks (instead of at breaks) of discrete scales to better visually separate the groups.
• Draw zebra (alternating dark/light) stripes along a date/time scale to indicate e.g. decades.
• Draw the minor grid lines for log scales like logarithmic graph paper.

Likewise, the strips can be seen as a guide for facets. Facets do a lot of things, ranging from the layout of panels to initiating scales, labelling and drawing strips to assembling the plot's gtable, etc. I think if we make the strips a guide, we can offload some of the facet's complexity onto the strip guide. Moreover, I think they might be a neat extension point, as you wouldn't have to rewrite the complicated methods of facets to draw a new type of strip. I've taken a similar approach here for your perusal of the concept.

[teunbrand]

I made a small proof-of-concept for panel grids as guides here.

[teunbrand]

Closing this as it probably doesn't make a lot of sense to implement this in vanilla ggplot2