label_ functions for degrees (temp/lat/long); superscript notation for axes

[bbolker]

Copied from here in case it's appropriate/welcome here. I'd be happy to make a PR if it's of interest.

Some miscellaneous functionality here for

• scientific notation with superscripts (i.e. 10^3, 10^4, etc.)
• latitude/longitude scales

These could be further improved by

• allowing for a LaTeX (intended for tikzDevice, e.g. $10^2$) vs plotmath output (e.g. parse(text="10^2"));
• reducing code duplication in lat/long, e.g. generating lat/long scales from a single factory function that determines whether to use E/W or N/S legends
• could further improve lat/long by allowing it to (optionally) express as degrees/minutes/seconds rather than decimal degrees
• make a temperature scale function

Does this functionality exist somewhere else in the gg-verse?

[dpseidel]

We have considered longitude and latitude labels in the past (#180) and previously supported a wrapper of number_format (now superseded by label_number) called degree_format (#178; now retired). In ggplot2, latitude and longitude scales are handled primarily by the sf package and the st_graticule function and so we haven't historically incorporated a standalone function here in scales. You can of course use the label_parse function or the label_number function to help format custom labels when working outside of a spatial object or with other plotting packages.

demo_continuous(c(49.67890), labels = label_number(suffix ="\u00b0"))
#> scale_x_continuous(labels = label_number(suffix = "°"))

graticule_labels <- c("180*degree", "160*degree*W", "140*degree*W")
demo_discrete(graticule_labels, labels = label_parse())
#> scale_x_discrete(labels = label_parse())

Scientific notation with superscripts is now supported by default with the label_math() function.

library(scales) #scales 1.1.0
demo_continuous(c(1, 2, 3, 4), labels = label_math())
#> scale_x_continuous(labels = label_math())

^{Created on 2020-02-22 by the reprex package (v0.3.0)}

[bbolker]

These are definitely good, it would be nice if they were more visible. A few minor points for discussion:

• For log scales, I was looking for something like this:

lm <- function(x) label_math()(log10(x))
demo_log10(10^(2:5),label=lm)

• My solution posted on the gist is a little bit fancier: the solution above doesn't work for other sets of breaks:

demo_log10(seq(100,1000,length=100)),label=lm)  ## ugly
## load scientific_10() from gist ...
demo_log10(seq(100,1000)) + scale_x_log10(label=scientific_10)  ## nicer

• does the unicode-based solution above work on Windows? (I don't have a system to test on before, but students of mine have encountered apparently unsurmountable problems with some Unicode characters [up- and down-arrows] on Windows ...)
• For my own use I would supplement or extend label_math with a version that would work with LaTeX formatting (for use with TikZ), but I recognize that may be a niche requirement ...

[teunbrand]

Small comment on the first point; the log scale you're proposing can be achieved by providing the log10() function as format argument to label_math(), i.e.:

demo_log10(10^(2:5), label = label_math(format = log10))

Though I agree a convenience wrapper would be nice.

[earthcli]

@dpseidel It is great. So how can I use this within scale_x_continuous(), for example, which is the right way to put the graticule_labels into scale_x_continuous ?
graticule_labels <- c("180degree", "160degreeW", "140degree*W")
scale_x_continuous(labels=label_parse())

[hadley]

Now a duplicate of #311, and will be resolved by #312.