Fix warning: aes_string() was deprecated in ggplot2 3.0.0

[xrobin]

Tests display the following warning on CI:

── Warning ('test-ggroc.R:9:2'): Ggroc screenshot looks normal ─────────────────
`aes_string()` was deprecated in ggplot2 3.0.0.
ℹ Please use tidy evaluation idioms with `aes()`.
ℹ See also `vignette("ggplot2-in-packages")` for more information.
Backtrace:
  1. pROC:::expect_doppelganger("ggroc.screenshot", test_ggplot_screenshot)
       at test-ggroc.R:9:8
  2. vdiffr::expect_doppelganger(title, fig, ...)
       at pROC-tests/testthat/helper-vdiffr.R:5:8
  3. vdiffr (local) writer(fig, testcase, title)
  5. vdiffr:::print_plot.function(plot, title)
  6. pROC (local) p()
  9. pROC:::ggroc.roc(...)
 10. pROC:::get.aes.for.ggplot(data, legacy.axes)
 12. ggplot2 (local) `<fn>`(x = "specificity", y = "sensitivity")
 13. ggplot2:::deprecate_soft0(...)

aes_string is used in the get.aes.for.ggplot function in ggroc.R to build the aesthetics. Alternatives are documented in Using ggplot2 in packages vignette.

Todo:

• Optional: find out how to reproduce the warning outside of CI
• Remove usage of aes_string in ggroc

[xrobin]

Solution as on StackOverflow.