tryCatch and ggplot

Question

I use tryCatch a lot to capture potential error messages in my R code, but I faced a strange issue today with ggplot - and I really do not have any idea what is going on (even after some time spent on Google/SO searching).

Simple example with an incomplete ggplot call (no added layer):

> tryCatch(eval(parse(text = 'ggplot(mtcars, aes(x=hp, y=wt))')), error = function(e) e)
Error: No layers in plot

So the error is not captured. Even more: I get an R object with ggplot class:

> str(tryCatch(eval(parse(text = 'ggplot(mtcars, aes(x=hp, y=wt))')), error = function(e) e))
List of 8
 $ data       :'data.frame':    32 obs. of  11 variables:
  ..$ mpg : num [1:32] 21 21 22.8 21.4 18.7 18.1 14.3 24.4 22.8 19.2 ...
  ..$ cyl : num [1:32] 6 6 4 6 8 6 8 4 4 6 ...
  ..$ disp: num [1:32] 160 160 108 258 360 ...
  ..$ hp  : num [1:32] 110 110 93 110 175 105 245 62 95 123 ...
  ..$ drat: num [1:32] 3.9 3.9 3.85 3.08 3.15 2.76 3.21 3.69 3.92 3.92 ...
  ..$ wt  : num [1:32] 2.62 2.88 2.32 3.21 3.44 ...
  ..$ qsec: num [1:32] 16.5 17 18.6 19.4 17 ...
  ..$ vs  : num [1:32] 0 0 1 1 0 1 0 1 1 1 ...
  ..$ am  : num [1:32] 1 1 1 0 0 0 0 0 0 0 ...
  ..$ gear: num [1:32] 4 4 4 3 3 3 3 4 4 4 ...
  ..$ carb: num [1:32] 4 4 1 1 2 1 4 2 2 4 ...
 $ layers     : list()
 $ scales     :Reference class 'Scales' [package "ggplot2"] with 1 fields
  ..$ scales: NULL
  ..and 20 methods, of which 9 are possibly relevant:
  ..  add, clone, find, get_scales, has_scale, initialize, input, n,
  ..  non_position_scales
 $ mapping    :List of 2
  ..$ x: symbol hp
  ..$ y: symbol wt
 $ options    :List of 1
  ..$ labels:List of 2
  .. ..$ x: chr "hp"
  .. ..$ y: chr "wt"
 $ coordinates:List of 1
  ..$ limits:List of 2
  .. ..$ x: NULL
  .. ..$ y: NULL
  ..- attr(*, "class")= chr [1:2] "cartesian" "coord"
 $ facet      :List of 1
  ..$ shrink: logi TRUE
  ..- attr(*, "class")= chr [1:2] "null" "facet"
 $ plot_env   :<environment: R_GlobalEnv> 
 - attr(*, "class")= chr "ggplot"

Any ideas to capture these kind of errors?

---

Update: I am aware of the fact that the above call might be OK in some situations (like adding a layer later). But then please consider the following example (please note that we have a typo there: t is provided on y axis instead of wt):

> tryCatch(eval(parse(text = 'ggplot(mtcars, aes(x=hp, y=t)) + geom_point()')), error = function(e) e)
Error in data.frame(x = c(110, 110, 93, 110, 175, 105, 245, 62, 95, 123,  : 
  arguments imply differing number of rows: 32, 0

I do get the error message, but that's not captured by tryCatch.

Answer 1

This happens because the error doesn't occur in the call to ggplot, but in the call to print.ggplot.

So your tryCatch works, but the error occurs only later, when trying to print. This means you have to put the tryCatch around the print statement:

Try this instead:

x <- tryCatch(eval(parse(text = 'ggplot(mtcars, aes(x=hp, y=wt))')), 
              error = function(e) e)
tryCatch(print(x), error = function(e) e)
<simpleError: No layers in plot>

---

And for your second example:

x <- eval(parse(text = 'ggplot(mtcars, aes(x=hp, y=t)) + geom_point()'))
tryCatch(print(x), error = function(e) e)

<simpleError in data.frame(x = c(110, 110, 93, 110, 175, 105, 245, 62, 95, 123, 123, 180, 180, 180, 205, 215, 230, 66, 52, 65, 97, 150, 150, 245, 175, 66, 91, 113, 264, 175, 335, 109), y = structure(function (x) standardGeneric("t"), generic = structure("t", package = "base"), package = "base", group = list(), valueClass = character(0), signature = "x", default = structure(function (x) UseMethod("t"), target = structure("ANY", class = structure("signature", package = "methods"), .Names = "x", package = "methods"), defined = structure("ANY", class = structure("signature", package = "methods"), .Names = "x", package = "methods"), generic = structure("t", package = "base"), class = structure("derivedDefaultMethod", package = "methods")), skeleton = structure(function (x) UseMethod("t"), target = structure("ANY", class = structure("signature", package = "methods"), .Names = "x", package = "methods"), defined = structure("ANY", class = structure("signature", package = "methods"), .Names = "x", package = "methods"), generic = structure("t", package = "base"), class = structure("derivedDefaultMethod", package = "methods"))(x), class = structure("standardGeneric", package = "methods")),     PANEL = c(1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L,     1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L,     1L, 1L, 1L, 1L, 1L), check.names = FALSE, stringsAsFactors = TRUE): arguments imply differing number of rows: 32, 0>