ggplot2 2.2.0

Major new features

Subtitle and caption

Thanks to @hrbrmstr plots now have subtitles and captions, which can be set with the subtitle and caption arguments to ggtitle() and labs(). You can control their appearance with the theme settings plot.caption and plot.subtitle. The main plot title is now left-aligned to better work better with a subtitle. The caption is right-aligned (@hrbrmstr).

Stacking

position_stack() and position_fill() now sort the stacking order to match grouping order. This allows you to control the order through grouping, and ensures that the default legend matches the plot (#1552, #1593). If you want the opposite order (useful if you have horizontal bars and horizontal legend), you can request reverse stacking by using position = position_stack(reverse = TRUE) (#1837).

position_stack() and position_fill() now accepts negative values which will create stacks extending below the x-axis (#1691).

position_stack() and position_fill() gain a vjust argument which makes it easy to (e.g.) display labels in the middle of stacked bars (#1821).

Layers

geom_col() was added to complement geom_bar() (@hrbrmstr). It uses stat="identity" by default, making the y aesthetic mandatory. It does not support any other stat_() and does not provide fallback support for the binwidth parameter. Examples and references in other functions were updated to demonstrate geom_col() usage.

When creating a layer, ggplot2 will warn if you use an unknown aesthetic or an unknown parameter. Compared to the previous version, this is stricter for aesthetics (previously there was no message), and less strict for parameters (previously this threw an error) (#1585).

Facetting

The facet system, as well as the internal panel class, has been rewritten in ggproto. Facets are now extendable in the same manner as geoms and stats, as described in vignette("extending-ggplot2").

We have also added the following new fatures.

• facet_grid() and facet_wrap() now allow expressions in their facetting
  formulas (@DanRuderman, #1596).
• When facet_wrap() results in an uneven number of panels, axes will now be
  drawn underneath the hanging panels (fixes #1607)
• Strips can now be freely positioned in facet_wrap() using the
  strip.position argument (deprecates switch).
• The relative order of panel, strip, and axis can now be controlled with
  the theme setting strip.placement that takes either inside (strip between
  panel and axis) or outside (strip after axis).
• The theme option panel.margin has been deprecated in favour of
  panel.spacing to more clearly communicate intent.

Extensions

Unfortunately there was a major oversight in the construction of ggproto which lead to extensions capturing the super object at package build time, instead of at package run time (#1826). This problem has been fixed, but requires re-installation of all extension packages.

Scales

• The position of x and y axes can now be changed using the position argument
  in scale_x_*and scale_y_* which can take top and bottom, and left
  and right respectively. The themes of top and right axes can be modified
  using the .top and .right modifiers to axis.text.* and axis.title.*.

Continuous scales

• scale_x_continuous() and scale_y_continuous() can now display a secondary
  axis that is a one-to-one transformation of the primary axis (e.g. degrees
  Celcius to degrees Fahrenheit). The secondary axis will be positioned opposite
  to the primary axis and can be controlled with the sec.axis argument to
  the scale constructor.
• Scales worry less about having breaks. If no breaks can be computed, the
  plot will work instead of throwing an uninformative error (#791). This
  is particularly helpful when you have facets with free scales, and not
  all panels contain data.
• Scales now warn when transformation introduces infinite values (#1696).

Date time

• scale_*_datetime() now supports time zones. It will use the timezone
  attached to the varaible by default, but can be overridden with the
  timezone argument.
• New scale_x_time() and scale_y_time() generate reasonable default
  breaks and labels for hms vectors (#1752).

Discrete scales

The treatment of missing values by discrete scales has been thoroughly overhauled (#1584). The underlying principle is that we can naturally represent missing values on discrete variables (by treating just like another level), so by default we should.

This principle applies to:

• character vectors
• factors with implicit NA
• factors with explicit NA

And to all scales (both position and non-position.)

Compared to the previous version of ggplot2, there are three main changes:

1. scale_x_discrete() and scale_y_discrete() always show discrete NA,
   regardless of their source
2. If present, NAs are shown in discete legends.
3. All discrete scales gain a na.translate argument that allows you to
   control whether NAs are translated to something that can be visualised,
   or should be left as missing. Note that if you don't translate (i.e.
   na.translate = FALSE) the missing values will passed on to the layer,
   which will warning that it's dropping missing values. To suppress the
   warnings, you'll also need to add na.rm = TRUE to the layer call.

There were also a number of other smaller changes

• Correctly use scale expansion factors.
• Don't preserve space for dropped levels (#1638).
• Only issue one warning when when asking for too many levels (#1674).
• Unicode labels work better on Windows (#1827).
• Warn when used with only continuous data (#1589)

Themes

• The theme() constructor now has named arguments rather than ellipses. This
  should make autocomplete substantially more useful. The documentation
  (including exampes) has been considerably improved.
• Built-in themes are more visually homogeneous, and match theme_grey better.
  (@jiho, #1679)
• When computing the height of titles, ggplot2 now includes the height of the
  descenders (i.e. the bits of g and y that hang beneath the baseline). This
  improves the margins around titles, particularly the y axis label (#1712).
  I have also very slightly increased the inner margins of axis titles, and
  removed the outer margins.
• Theme element inheritance is now easier to work with as modification now
  overrides default element_blank elements (#1555, #1557, #1565, #1567)
• Horizontal legends (i.e. legends on the top or bottom) are horizontally
  aligned by default (#1842). Use legend.box = "vertical" to switch back
  to the previous behaviour.
• element_line() now takes an arrow argument to specify arrows at the end of
  lines (#1740)

There were a number of tweaks to the theme elements that control legends:

• legend.justification now controls appearance will plotting the legend
  outside of the plot area. For example, you can use
  theme(legend.justification = "top") to make the legend align with the
  top of the plot.
• panel.margin and legend.margin have been renamed to panel.spacing and
  legend.spacing respectively, to better communicate intent (they only
  affect spacing between legends and panels, not the margins around them)
• legend.margin now controls margin around individual legends.
• New legend.box.background, legend.box.spacing, and legend.box.margin
  control the background, spacing, and margin of the legend box (the region
  that contains all legends).

Bug fixes and minor improvements

• ggplot2 now imports tibble. This ensures that all built-in datasets print
  compactly even if you haven't explicitly loaded tibble or dplyr (#1677).
• Class of aesthetic mapping is preserved when adding aes() objects (#1624).
• +.gg now works for lists that include data frames.
• annotation_x() now works in the absense of global data (#1655)
• geom_*(show.legend = FALSE) now works for guide_colorbar.
• geom_boxplot() gains new outlier.alpha (@jonathan-g) and
  outlier.fill (@schloerke, #1787) parameters to control the alpha/fill of
  outlier points independently of the alpha of the boxes.
• position_jitter() (and hence geom_jitter()) now correctly computes
  the jitter width/jitter when supplied by the user (#1775, @has2k1).
• geom_contour() more clearly describes what inputs it needs (#1577).
• geom_curve() respects the lineend paramater (#1852).
• geom_histogram() and stat_bin() understand the breaks parameter once
  more. (#1665). The floating point adjustment for histogram bins is now
  actually used - it was previously inadvertently ignored (#1651).
• geom_violin() no longer transforms quantile lines with the alpha aesthetic
  (@mnbram, #1714). It no longer errors when quantiles are requested but data
  have zero range (#1687). When trim = FALSE it once again has a nice
  range that allows the density to reach zero (by extending the range 3
  bandwidths to either side of the data) (#1700).
• geom_dotplot() works better when facetting and binning on the y-axis.
  (#1618, @has2k1).
• geom_hexbin() once again supports ..density.. (@mikebirdgeneau, #1688).
• geom_step() gives useful warning if only one data point in layer (#1645).
• layer() gains new check.aes and check.param arguments. These allow
  geom/stat authors to optional suppress checks for known aesthetics/parameters.
  Currently this is used only in geom_blank() which powers expand_limits()
  (#1795).
• All stat_*() display a better error message when required aesthetics are
  missing.
• stat_bin() and stat_summary_hex() now accept length 1 binwidth (#1610)
• stat_density() gains new argument n, which is passed to underlying function
  stats::density ("number of equally spaced points at which the
  density is to be estimated"). (@hbuschme)
• stat_binhex() now again returns count rather than value (#1747)
• stat_ecdf() respects pad argument (#1646).
• stat_smooth() once again informs you about the method it has chosen.
  It also correctly calculates the size of the largest group within facets.
• x and y scales are now symmetric regarding the list of
  aesthetics they accept: xmin_final, xmax_final, xlower,
  xmiddle and xupper are now valid x aesthetics.
• Scale extensions can now override the make_title and make_sec_title
  methods to let the scale modify the axis/legend titles.