[vsimko]

What this changes

The documentation for transKeepSign was cut off mid-sentence — the manual
ended at NOTE: If FALSE, the non-corr matrix will be, which is where the
roxygen block in R/corrplot.R also stops. So the argument was documented by
a sentence that never finished.

This replaces the note with an actual description:

• With transKeepSign = TRUE (default), values are divided by
  max(abs(col.lim)). They keep their sign and zero stays at the midpoint of
  the palette, so a diverging palette is centred on zero just as it is for a
  correlation matrix. This is the one to use when zero is a meaningful
  reference (differences, log ratios, z-scores).
• With transKeepSign = FALSE, values are rescaled linearly so that
  col.lim[1] takes the first colour and col.lim[2] the last: the whole
  palette is used over the whole range, at the cost of zero no longer sitting
  in the middle. A sequential palette usually fits better there.
• Also documented two behaviours that weren't mentioned anywhere: the
  value-based methods (circle, square, ellipse, shade, pie) are
  rejected when the sign is dropped, because the rescaled values are all
  non-negative; and a matrix whose values are all positive or all negative is
  always rescaled linearly over col.lim, whatever the flag says.

No behaviour changes — documentation and tests only.

Files touched

• R/corrplot.R, man/corrplot.Rd — the rewritten argument description, plus
  an example that plots a non-correlation matrix with col.lim = c(-8, 8)
  both ways.
• vignettes/example-corrplot.R — the same example for the vignette.
• tests/testthat/test-corrplot.R — regression test for the documented
  behaviour: value-keeping with TRUE, (x - col.lim[1]) / diff(col.lim) with
  FALSE, the error for glyph methods with FALSE, and the identity for a
  single-sign matrix.
• NEWS.md — entry under the development version.

Checks

R CMD INSTALL .
testthat::test_dir("tests/testthat")
#> Failed: 0, Warnings: 0, Skipped: 8

(The 8 skips are pre-existing empty test files.)

Fixes #287.