explain test conditional in news + update one visual regression test

Author: corybrunson

NEWS.md

@@ -4,6 +4,10 @@
 
 The paramater `outline.type` from `ggplot2::geom_ribbon()` has been adapted to `geom_alluvium()` and `geom_flow()` with the same options and default.
 
+## remove test dependency on {alluvial}
+
+The tests that use the `Refugees` data set from **alluvial** are now only run if that package is installed; see #131 and thanks to @MichaelChirico.
+
 # ggalluvial 0.12.5
 
 This patch fixes a bug involving the {dplyr} functions `first()` and `last()` that was shifted but not fixed in 0.12.4. Rather than being imported during build, they are accessed internally and thus imported during use. See issues #107 <https://github.com/corybrunson/ggalluvial/issues/107> and #108 <https://github.com/corybrunson/ggalluvial/issues/108> on GitHub for details.
@@ -12,7 +16,7 @@ This patch fixes a bug involving the {dplyr} functions `first()` and `last()` th
 
 ## `linewidth` aesthetic (breaking change)
 
-An upcoming release of *ggplot2* controls stroke width using the new `linewidth` aesthetic rather than `size`. This release adapts to this change internally by updating row and column layers as recommended here: <https://www.tidyverse.org/blog/2022/08/ggplot2-3-4-0-size-to-linewidth/>
+An upcoming release of **ggplot2** controls stroke width using the new `linewidth` aesthetic rather than `size`. This release adapts to this change internally by updating row and column layers as recommended here: <https://www.tidyverse.org/blog/2022/08/ggplot2-3-4-0-size-to-linewidth/>
 
 ## Curve constructors
 

tests/testthat/_snaps/geom-lode/geom-lode-one-axis.svg

@@ -3,14 +3,10 @@
 test_that("`geom_alluvium` draws correctly", {
   skip_on_cran()
   skip_if_not_installed("vdiffr")
-  skip_if_not_installed("alluvial")
 
   d1 <- as.data.frame(Titanic)
   a1 <- aes(y = Freq, axis1 = Class, axis2 = Sex, axis3 = Age, fill = Survived)
   a2 <- aes(y = Freq, axis1 = Class, axis2 = Sex)
-  d2 <- alluvial::Refugees
-  a3 <- aes(y = refugees, x = year, alluvium = country)
-  
   vdiffr::expect_doppelganger(
     "`geom_alluvium`: basic",
     ggplot(d1, a1) + geom_alluvium()
@@ -21,6 +17,11 @@ test_that("`geom_alluvium` draws correctly", {
       geom_alluvium(aes(fill = Age), width = .4) +
       facet_wrap(~ Survived, scales = "fixed")
   )
+  
+  skip_if_not_installed("alluvial")
+  
+  d2 <- alluvial::Refugees
+  a3 <- aes(y = refugees, x = year, alluvium = country)
   vdiffr::expect_doppelganger(
     "`geom_alluvium`: bump plot",
     ggplot(d2, a3) +