Wrong x range when using coord_cartesian() with zero range xlim and expand=FALSE

[wch]

When coord_cartesian() is called a zero-range xlim and expand=FALSE, the x value appears to be treated as a factor, and all points are in the plot.

ggplot(mtcars, aes(wt, mpg)) +
  geom_point() +
  coord_cartesian(xlim = c(5, 5), expand = FALSE)

[thomasp85]

I agree the current situation is not good, but how would you envision showing a zero-range scale with no expansion?

[wch]

Hm, that's a good question. Maybe it would be reasonable to use a tiny value for expand?

For a context, this issue came up in this shiny app: https://gallery.shinyapps.io/105-plot-interaction-zoom/
In the middle plot, if you select a zero-width rectangle, the right plot looks like the one I included above. It's easiest to create a zero-width rectangle if you first create a rectangle (and release the mouse button), then drag one of the sides to meet the other one.

[yutannihilation]

Maybe it would be reasonable to use a tiny value for expand?

I understand it would be useful if ggplot2 does this, but I think it's not easy for a coord to know how tiny is tiny enough.

expand = FALSE + zero-width range can be detected when setting view scales:

ggplot2/R/layout.R

Lines 199 to 213 in 6f5ffea

	setup_panel_params = function(self) {
	# Fudge for CoordFlip and CoordPolar - in place modification of
	# scales is not elegant, but it is pragmatic
	self$coord$modify_scales(self$panel_scales_x, self$panel_scales_y)
	scales_x <- self$panel_scales_x[self$layout$SCALE_X]
	scales_y <- self$panel_scales_y[self$layout$SCALE_Y]
	setup_panel_params <- function(scale_x, scale_y) {
	self$coord$setup_panel_params(scale_x, scale_y, params = self$coord_params)
	}
	self$panel_params <- Map(setup_panel_params, scales_x, scales_y)
	invisible()
	},

ggplot2/R/coord-cartesian-.r

Lines 99 to 104 in 6f5ffea

	setup_panel_params = function(self, scale_x, scale_y, params = list()) {
	c(
	view_scales_from_scale(scale_x, self$limits$x, self$expand),
	view_scales_from_scale(scale_y, self$limits$y, self$expand)
	)
	},

or more precisely here:

ggplot2/R/scale-expansion.r

Lines 156 to 158 in 6f5ffea

	expand_limits_continuous <- function(limits, expand = expansion(0, 0), coord_limits = c(NA, NA)) {
	expand_limits_continuous_trans(limits, expand, coord_limits)$continuous_range
	}

So, we might be able to add some expansion here, but, since Layout and Coord have no acccess to the data (or the range of the data) here, the proper width cannot be determined.

On the other hand, a view scale can know both the range and the data on rendering, so resolution(x) can be used for adjusting the limit.

ggplot2/R/scale-view.r

Lines 130 to 132 in 6f5ffea

	rescale = function(self, x) {
	self$scale$rescale(x, self$limits, self$continuous_range)
	},

But, the problem is it's difficult to warn with a meaningful message because a view scale doesn't know it's context, iirc...

# c.f. https://github.com/tidyverse/ggplot2/commit/16e34032d1951f51c31d8fd109c4fb5bb3622444
devtools::load_all("~/repo/ggplot2")
#> Loading ggplot2

ggplot(mtcars, aes(wt, mpg)) +
  geom_point() +
  coord_cartesian(xlim = c(5, 5), expand = FALSE)
#> Warning: A zero-width range is specified for coord

#> Warning: A zero-width range is specified for coord

#> Warning: A zero-width range is specified for coord

#> Warning: A zero-width range is specified for coord

^{Created on 2019-10-27 by the reprex package (v0.3.0)}

[teunbrand]

Just as a small note, the rescale() function declares anything with zero-range to be mean(to), which is why all points show up halfway the x-axis in the first example.

scales::rescale(1:10, to = c(0, 1), from = c(5, 5))
#>  [1] 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5

^{Created on 2024-02-20 with reprex v2.1.0}