Issue 161: by column with plot_grid

[BrianLang]

Fix #161

• Added argument bycol to function call.
• Calculate plot ordering if bycol is TRUE.
• Moved grobs calculation to after plots ordering.

[clauswilke]

I'm fine with this in principle, but this PR will need a lot of work before it can be merged. Just off the top of my head:

• Call the argument byrow as in matrix().

• Place the new argument before the deprecated arguments.

• Don't use F, write FALSE.

• The new argument needs to be documented and documentation re-generated.

• There needs to be an entry in the changelog (NEWS.md). Look at the ggplot2 source for how to format contribution by outside contributors.

• There needs to be a visual test for the new feature. Visual tests are currently failing, so I need to fix them first. I suggest you develop the code for the visual test but just post it here in a comment rather than adding to the code at this time, so we can discuss.

[BrianLang]

Thanks for this feedback, I now have done most of these things to the extent that I understand what you want.

• I've updated the argument to be byrow.
• I've moved the argument to be before rows and cols.
• I've used FALSE instead of F
• I've written argument documentation and an example and re-generated the documentation.
• I added this to the NEWS.md:

  Minor changes

  • plot_grid() now accepts byrow as an argument to place plots by row or by column onto the plot area. (@BrianLang, #162).

• For a visual test I think we would add this to the test_that("basic plot arranging works" test:

  expect_doppelganger("colwise arranging",
                    plot_grid(p1, NULL, p2, NULL, p3, NULL, byrow = FALSE) + theme_map()
  )