contour fails when coordinates are not aligned with axes

[cneyens]

Below I have regularly spaced points that are not aligned with the plot axes. I want to plot contour lines but geom_contour seems to fail here. I vaguely remember this used to work in some former ggplot version, but I tried with versions 3.3.2 and 3.3.0 and those also don't plot contours for this data set so I might be wrong... I would expect geom_contour to be able to handle this since the points are fairly regularly spaced (disregarding minor rounding errors from the rotation calculation I've done here).

For context: these could be spatial data points (XY coordinates) placed on a regular grid for which I've obtained several continuous variables, e.g. groundwater levels and chloride concentrations.

library(ggplot2)

df <- expand.grid(x = 1:10,
                  y = 1:10)
df$z <- c(volcano[53:62, 29:38]) # arbitrary

ggplot(df, aes(x = x, y = y)) +
  geom_point() +
  geom_contour(aes(z = z)) +
  coord_fixed()

# rotate points 15 degrees counterclockwise
angle <- atan(df$y/df$x) * 180/pi + 15
df$rotx <- cos(angle * pi/180) * sqrt(df$x^2 + df$y^2)
df$roty <- sin(angle * pi/180) * sqrt(df$x^2 + df$y^2)

p <- ggplot(df, aes(x = rotx, y = roty)) +
  geom_point() +
  coord_fixed()

p + geom_contour(aes(z = z))
#> Warning: stat_contour(): Zero contours were generated
#> Warning in min(x): no non-missing arguments to min; returning Inf
#> Warning in max(x): no non-missing arguments to max; returning -Inf

^{Created on 2021-01-20 by the reprex package (v0.3.0)}

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[clauswilke]

The requirement for contouring is not that you have a regularly spaced grid but that you have a matrix of z values whose locations can be identified with marginal vectors of x and y values.

See here for a grid that is not regularly spaced:

library(ggplot2)

df <- expand.grid(x = c(1, 2, 4, 8, 16, 32),
                  y = c(1, 2, 4, 8, 16, 32))
df$z <- c(volcano[53:58, 29:34]) # arbitrary

ggplot(df, aes(x = x, y = y)) +
  geom_point() +
  geom_contour(aes(z = z)) +
  coord_fixed()

^{Created on 2021-01-20 by the reprex package (v0.3.0)}

If your input data doesn't have this structure then you may have to force it into this structure via interpolation.

[clauswilke]

(Having said this, it would probably be possible to add this feature to geom_contour(), so I consider this a feature request.)

[clauswilke]

Relevant SO post with a fairly simple solution: https://stackoverflow.com/a/38177092/4975218

Requires an additional dependency on the akima package: https://cran.r-project.org/web/packages/akima/index.html

[cneyens]

Thanks!

I'm aware of multiple SO posts where interpolation is offered as a solution for cases where the starting data set is not gridded, but I was under the impression that the requirement for geom_contour to work was that the data should be (roughly) evenly spaced as stated in the docs. This clears things up for me:

The requirement for contouring is not that you have a regularly spaced grid but that you have a matrix of z values whose locations can be identified with marginal vectors of x and y values.

[clauswilke]

If you feel the documentation is not sufficiently clear, could you open a separate issue just about that?