TLC for polar coordinates: `coord_radial()` #5334

teunbrand · 2023-06-25T12:27:26Z

This PR fixes #4815, fixes #5059, fixes #3959, and fixes #4462

Briefly, this PR introduces a new coord system coord_polar2() with expanded options (name of function and arguments are up for discussion).

Less briefly, this PR started out with a simpler goal of having coord_polar() use the axis system. Realising that this couldn't be done without breaking backward compatibility, this has instead become coord_polar2(). This PR then started accumulating additional features and here we are today. I think it might be best to explain this PR with a visual walkthrough.

If we use coord_polar2() plainly, we see it resembles coord_polar() with the following differences:

Theta is represented by a proper position axis. Figuring out text placement here was the hardest part, and still isn't perfect. The default is to display text of the theta axis horizontally.
The panel.background theme setting is applied to the circle that forms the background. You can still use panel.border = element_rect(fill = NA) in the theme if you need a frame for the plot.
Radius axis line starts and stops at the actual radius limits and doesn't cover the entire left side of the rectangular panel.
The coords has a expand = TRUE default argument so that the behaviour of scale expansion is more in line with coord_cartesian() than with coord_polar().

devtools::load_all("~/packages/ggplot2/")
#> ℹ Loading ggplot2

p <- ggplot(mtcars, aes(disp, mpg)) +
  geom_point() +
  theme(axis.line = element_line())

p + coord_polar2()

One feature is that we can also place the radius axis inside the circle. Admittedly, this looks a little bit awkward because it intersects with the theta axis, which is why this isn't the default when you have a full circle.

p + coord_polar2(r_axis_inside = TRUE)

The next feature is that requested in #4462, namely to have partial polar coordinates. The following things are of note:

The start and end arguments to control which sector of a circle the plot occupies.
Since we don't have a full circle, the radius axis is placed inside and not at the rectangular bounding box. You can use coord_polar2(r_axis_inside = FALSE) to prevent this.
The radius axis snaps to the start position. While the axis itself is rotated to reflect this, the labels remain horizontal.

p + coord_polar2(start = -0.25 * pi, end = 0.25 * pi)

If you swap direction, the radius axis follows the swap.

p + coord_polar2(start = -0.25 * pi, end = 0.25 * pi, direction = -1)

Then the reason I started this PR: #3959. A few comments:

There is a new guide_axis_theta() that can be used for the theta axis.
Radius axes cannot be the new theta guide, which is what prompts the warning below. The inverse is also true, i.e. guides(theta = "axis") also throws a warning.
If the angle argument is set, either in a radius axis or theta axis, this is interpreted as a relative angle. Note that the theta text follows the curvature and that the radius text are projected from the tick marks.

p + coord_polar2(start = -0.25 * pi, end = 0.25 * pi) +
  guides(
    r     = guide_axis_theta(),
    r.sec = guide_axis(angle = 0),
    theta = guide_axis_theta(angle = 0)
  )
#> Warning: `guide_axis_theta()` cannot be used for r.
#> ℹ Use one of x, y, or theta instead.

I then took a little bit of a liberty and decided on my own it would be fun to also be able to set a donut hole in polar coordinates. This is mostly just a convenience that doesn't force you to fiddle with the scale limits or expansions to not have all shapes disappear in a single point at the center. Also, it is the only circumstance I could ever imagine that you'd need a secondary theta axis.

p + coord_polar2(donut = 0.5) +
  guides(theta.sec = "axis_theta")

Lastly, I also implemented #5059, since getting the (relative) angle right in text geoms in polar coordinates is a bit of pain. Text between 90 and 270 degrees is flipped for readability reasons.

df <- data.frame(
  x = LETTERS[1:5], lab = c("cat", "farm", "banana", "airplane", "baker")
)

ggplot(df, aes(x, label = lab)) +
  geom_text(aes(y = "0 degrees"),  angle = 0)  +
  geom_text(aes(y = "90 degrees"), angle = 90) +
  coord_polar2(rotate_angle = TRUE)

^{Created on 2023-06-25 with reprex v2.0.2}

A few closing remarks:

guide_axis_theta() also works reasonably well with cartesian coordinates.
guide_axis_theta() does not respond to hjust/vjust settings, mostly because text placement had given me enough headaches at this point and the theme's defaults translate horribly to the theta axis.
The theta positions uses x.bottom theme settings, theta.sec uses x.top, r/r.sec use y.left/y.right depending on the direction argument. In theory, we could change this, but it would involve some hairy adaptations to guide_axis() as well.
Clipping currently isn't ideal, as it applies to the rectangular bounding box. We could use R4.1+ features to clip to the curved bounding boxes, but we'd need to implement Feature request: graphics device capabilities checker #5332 for that.

…nto polar_guides

teunbrand · 2023-06-25T12:28:21Z

R/guide-axis-theta.R

+GuideAxisTheta <- ggproto(
+  "GuideAxisTheta", GuideAxis,
+
+  # TODO: delete if minor ticks PR (#5287) gets merged


This may be simplified once #5287 is merged.

teunbrand · 2023-06-25T12:29:39Z

R/guide-axis.R

 #' # can also be used to add a duplicate guide
 #' p + guides(x = guide_axis(n.dodge = 2), y.sec = guide_axis())
-guide_axis <- function(title = waiver(), check.overlap = FALSE, angle = NULL,
+guide_axis <- function(title = waiver(), check.overlap = FALSE, angle = waiver(),


The default is now waiver() because coord_polar2() may overrule angle for display purposes, but NULL still uses the theme's angle.

teunbrand · 2023-06-25T12:30:34Z

R/guide-axis.R

  }

-  # it is not worth the effort to align upside-down labels properly
-  check_number_decimal(angle, min = -90, max = 90)


I very much did need some alignments for upside-down labels, so I have expanded this function to include this. Due to this, some angles in the svg snapshots have changed from e.g. 45 degrees to -315 degrees but that doesn't matter visually.

teunbrand · 2023-06-25T12:31:39Z

R/layout.R

-      position  <- params[[1]]$position %||% scale$position
-      if (position != scale$position) {
-        order <- rev(order)
+      if (!is.null(params)) {


Layout doesn't know about theta/r guides, so had to avoid taking labels from NULL params here that are returned when the x/y guide doesn't exist.

teunbrand · 2023-06-25T12:46:40Z

Also worth pointing out that interactions with e.g. {ggtext} aren't that horrible:

devtools::load_all("~/packages/ggplot2/")
#> ℹ Loading ggplot2
library(ggtext)
labels <- c(
  setosa = "<img src='https://upload.wikimedia.org/wikipedia/commons/thumb/8/86/Iris_setosa.JPG/180px-Iris_setosa.JPG'
    width='100' /><br>*I. setosa*",
  virginica = "<img src='https://upload.wikimedia.org/wikipedia/commons/thumb/3/38/Iris_virginica_-_NRCS.jpg/320px-Iris_virginica_-_NRCS.jpg'
    width='100' /><br>*I. virginica*",
  versicolor = "<img src='https://upload.wikimedia.org/wikipedia/commons/thumb/2/27/20140427Iris_versicolor1.jpg/320px-20140427Iris_versicolor1.jpg'
    width='100' /><br>*I. versicolor*"
)

ggplot(iris, aes(Species, Sepal.Width)) +
  geom_boxplot() +
  scale_x_discrete(
    name = NULL,
    labels = labels
  ) +
  coord_polar2(r_axis_inside = TRUE) +
  theme(
    axis.text.x = element_markdown(color = "black", size = 11),
    axis.title.y = element_blank(),
    plot.margin = margin(5,5,70,5)
  )

^{Created on 2023-06-25 with reprex v2.0.2}

Note that you couldn't do this with regular coord_polar().

R/coord-polar2.R

R/guide-axis-theta.R

thomasp85 · 2023-10-04T06:24:33Z

Name suggestion: coord_radial()

teunbrand · 2023-10-25T09:08:20Z

Latest changes:

Renamed coord_polar2() to coord_radial().
Use minor ticks plumbing from Minor ticks #5287, so custom extract_key method and params became redundant.
coord_polar() and coord_radial() now share the rescale helper functions

Should be ready for review :)

teunbrand · 2023-10-25T11:30:41Z

I couldn't really use Map() with old R (3.6.3) units to render labels, so instead I just vectorised rotate_just(). This allows element_text() to have multiple angles, thereby eliminating the need for Map()ing the labels.

thomasp85

I approve this, but please fix the small style stuff I commented on

R/guide-axis-theta.R

thomasp85 · 2023-11-20T10:52:15Z

R/guide-axis-theta.R

+      # We likely have a linear coord, so we match the text angles to
+      # standard axes.


What happens if this axis is used with a non-radial coord?

It will draw a very normal looking axis with slightly inferior options for text justification.

devtools::load_all("~/packages/ggplot2") #> ℹ Loading ggplot2 ggplot(mpg, aes(displ, hwy)) + geom_point() + guides(x = "axis_theta", y = "axis_theta") + theme(axis.line = element_line())

^{Created on 2023-11-20 with reprex v2.0.2}

R/guide-axis-theta.R

teunbrand · 2023-11-20T14:52:59Z

Thanks for the review Thomas!

teunbrand added 16 commits June 19, 2023 21:59

allow alignment of upside-down labels

9c7d4be

allow alignment of upside-down labels

e698b13

Merge branch 'polar_guides' of https://github.com/teunbrand/ggplot2 i…

d33fb9d

…nto polar_guides

Allow angle to be a waiver

d5fce8c

First implementation of guide_axis_theta

0f11fd3

Add theta/r to available aes

25bdb66

theta guide refuses regular positions

c21ac20

Don't set labels to NULL when guide cannot be found

ee65aba

opposite_position helper

07a625a

Fallback for regular positions

baa591a

Simplify tickmarks

3b7159b

regular axis sets opposite aesthetic

94a2ad3

accommodate secondary theta scale

a32c6cb

First implementation of coord_polar2

b7b9770

Polishes

36c93ee

Write tests

a1d980b

teunbrand commented Jun 25, 2023

View reviewed changes

teunbrand mentioned this pull request Jun 25, 2023

Partial polar plots #5028

Closed

teunbrand added coord 🗺️ feature a feature request or enhancement labels Jul 9, 2023

thomasp85 reviewed Aug 7, 2023

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teunbrand added 4 commits October 25, 2023 09:19

Merge branch 'main' into polar_guides

11b6811

update minor ticks implementation

67feb63

Rename file

5acb8ab

rename coord_polar2 -> coord_radial

c159487

teunbrand added 2 commits October 25, 2023 10:48

unify helpers

c0c3fa7

document

be1653a

teunbrand changed the title ~~TLC for polar coordinates~~ TLC for polar coordinates: coord_radial() Oct 25, 2023

teunbrand added 5 commits October 25, 2023 11:10

Add theta guide topic

42cfa21

Drop minor ticks example

f84f41b

vectorise rotate_just()

3c97b98

Don't Map() labels

d7e9a2d

accept snapshot

076035b

Clean up references to CoordPolar2

0c03a19

teunbrand added this to the ggplot2 3.5.0 milestone Nov 7, 2023

thomasp85 approved these changes Nov 20, 2023

View reviewed changes

teunbrand and others added 3 commits November 20, 2023 14:38

comments and remove return statements

16653fe

Merge branch 'main' into polar_guides

0ae805f

add news bullet

9ca4b65

teunbrand merged commit 5e29f33 into tidyverse:main Nov 20, 2023

		# We likely have a linear coord, so we match the text angles to
		# standard axes.

TLC for polar coordinates: coord_radial() #5334

TLC for polar coordinates: coord_radial() #5334

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TLC for polar coordinates: `coord_radial()` #5334

TLC for polar coordinates: `coord_radial()` #5334

teunbrand commented Jun 25, 2023 •

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