Formula extensions

[zeileis]

Fixes #534

With this PR tinyplot allows formulas without x variable, e.g., as in ~ 1 or ~ 0 or y ~ 1 or y ~ 0 etc. All of these are translated to x = NULL in the default method call.

df <- data.frame(x0 = c(0, .1), y0 = c(.2,1), x1 = c(1, .9), y1 = c(.75, 0))
tinyplot(~ 0, xmin=x0, ymin=y0, xmax=x1, ymax=y1, data = df, type = "segments")

Note that the axis labels are not ideal, though. I wasn't sure where this would best be sanitized.

[zeileis]

When checking how the type would be handled for x = NULL, I noticed that the type sanitizer currently only handles bivariate y ~ x plots:

https://github.com/grantmcdermott/tinyplot/blob/main/R/sanitize_type.R#L45-L55

I would recommend that we add support for the following:

• x = <factor>, y = NULL: barplot
• x = NULL, y = <factor>: barplot
• x = NULL, y = !<factor>: scatterplot against index
• x = !<factor>, y = NULL: histogram

Note, however, that the latter two are not consistent with the base R plot() function. But maybe that's ok, given that base R's formula and default method are not even consistent here. For plot(~ x) a stripchart() is generated (which I find far less useful than a histogram) and for plot(x) a scatterplot against the index is generated.

[grantmcdermott]

Note, however, that the latter two are not consistent with the base R plot() function. But maybe that's ok, given that base R's formula and default method are not even consistent here.

Thanks @zeileis and I agree. We should follow base plot conventions where possible, but be fine to deviate in cases like this where it makes sense. Do you want to tackle these univariate cases in this PR, or a separate one?

Everything else looks good, except the axis label as you say. I'd need to play around with the code a bit to see the best place to fix.

[zeileis]

Thanks for the feedback! I think it makes sense to include the univariate defaults in this PR because we can think about which formulas map to which x/y specifications which in turn map to plot types. I'll try to post a suggestion later today...