Facet to-do list

[grantmcdermott]

Moved from #83.

• Smarter arrangements of facets (e.g., "square" window arrangements) instead of current single row default. (Facet args (and some extras) #91)
• Smarter axes options (e.g., only print the axis ticks for the "outer" facets). (Facet args (and some extras) #91)
• Should we allow free axis scaling/limits for individual facets? Decision: No. (For now at least; see discussion and MWE in reply below.)
• Extra facets.args argument to control the above three options (maybe others). (Facet args (and some extras) #91)
• Support facet formula #100
• If yes to the above, should we integrate formula support directly into the plot.formula method somehow... maybe via a second vertical bar, e.g. y ~ x | by | facetrow ~ facetcol? Decision: No.
• Alternatively, maybe y ~ x | by, facet = TRUE could treat by as a facet variable and override the default grouping behaivour (distinct from facet = "by", which treats them as equivalent). Decision: No.
• Should the larger legend cex be adjusted to match the smaller facet elements (or, vice versa)? (Facet args (and some extras) #91)
• Consistent facet margins that can be adjusted by the user. (Legend and facet spacing #94)
• Customizable facet strip text. #98
• facet formulas don't play nice with type = "density" #103

Relevant comments:

#83 (comment) (@zeileis)

Layout: In my first attempt to add both faceting and categorical variables #12, I had just added an argument mfrow so that we could set mfrow = TRUE or mfrow = c(1, 4) etc. Maybe this would be feasible for letting the users decide whether they want square layouts or not.

Axes: It would be good to separate out the functions for drawing the panels and drawing the axes now. This might let us add the support for categorical variables from #12 that we had further discussed in #2. My suggestion was that there should be workhorse functions a la plot2_xclass_yclass_plottype(x, y, by, axes = TRUE, ...).

From reading your description above (but not studying the code in detail), I thought that it might maybe help to have accompanying functions axis2_xlass_yclass_plottype(...) that could be inserted into the workflow for drawing the axes?

#83 (comment) (@vincentarelbundock)

Personally, I would avoid putting too much burden on the formula. Faceting does feel like a different functionality which should mostly (exclusively?) be called with its own argument. Also, focusing on the facet argument eventually allow us to do more complex things like multiple variables with a two-sided formula indicating rows and columns.

The mfrow feature seems important and powerful. Is that name too generic? Will people think it does something else, like allow multiple plot2() calls? If it only applies to facet, maybe it should be facet_mfrow. I don't have a strong view...

[grantmcdermott]

Should we allow free axis scaling/limits for individual facets?

This is possible to do. But it ends up requiring more work internally than I initially anticipated. Reason: We have to manually reset par("usr") when drawing the plot elements of each individual facet at the end of tinyplot(). If we don't reset par("usr"), then each facet's extent is determined by the final facet plot region that was drawn higher up in the code logic. (Remember: We have to draw all of the plot regions before the drawing all the plot elements, otherwise we can't get facets to play nice with grouping.)

For posterity, I just tested a MWE with a new facet.args(... free = TRUE) argument that adds two pieces of code logic:

1. First, inside the chunk starting at l. 917.

if (nfacets > 1) {
  # ...  [existing code]
 
   if (isTRUE(facet.args[["free"]])) {
      xlim = range(split(x, facet)[[ii]])
      ylim = range(split(y, facet)[[ii]])
      if (!is.null(ymin)) ylim = range(ylim, range(split(ymin, facet)[[ii]]))
      if (!is.null(ymax)) ylim = range(ylim, range(split(ymax, facet)[[ii]]))
      frame.plot = TRUE
   }
}

2. Second, inside the chunk starting at l. 1150

if (nfacets > 1) {
  # ...  [existing code]

   if (isTRUE(facet.args[["free"]])) {
      xlim = range(xx)
      ylim = range(yy)
      if (!is.null(yymin)) ylim = range(ylim, range(yymin))
      if (!is.null(yymax)) ylim = range(ylim, range(yymax))
      xlim = extendrange(range(mtcars$wt),f=0.04)
      par(usr = c(extendrange(xlim), extendrange(ylim)))
   }
}

This works:

pkgload::load_all("~/Documents/Projects/tinyplot")
#> ℹ Loading tinyplot

tpar(facet.bg = "grey90")

plt(
  mpg ~ wt, mtcars,
  facet = cyl ~ am,
  facet.args = list(free = TRUE), ## TEST
  main = "Free facet axes scales/limits (example)"
)

^{Created on 2024-03-09 with reprex v2.1.0}

Buuut.... it seems fragile: I really don't like messing with par("usr") manually and I'm not sure that it's worth the effort. So, I'm marking as free axis scales as "wont support" for the moment. We can revisit if there's strong user demand for it.