Prepare release notes

@thomasp85 can you please take a quick look

Author: hadley

vignettes/news/2-2-0.Rmd

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+# ggplot2 2.2.0 coming soon!
+
+```{r setup, include=FALSE}
+library(ggplot2)
+library(dplyr)
+library(forcats)
+
+knitr::opts_chunk$set(
+  fig.asp = 1 / 1.6,
+  out.width = "75%",
+  collapse = TRUE,
+  comment = "#>"
+)
+```
+
+I'm planning to release ggplot2 2.2.0 in early November. In preparation, I'd like to announce that a release candidate is now available: version 2.1.0.9001. Please try it out, and file an [issue on GitHub](https://github.com/hadley/ggplot2/issues) if you discover any problems. I hope we can find and fix any major issues before the official release.
+
+Install the pre-release version with:
+
+```{r, eval = FALSE}
+# install.packages("devtools")
+devtools::install_github("hadley/ggplot2")
+```
+
+If you discover a major bug that breaks your plots, please [file a minimal reprex](https://github.com/hadley/ggplot2/issues), and then roll back to the released version with:
+
+```{r, eval = FALSE}
+install.packages("ggplot2")
+```
+
+ggplot2 2.2.0 will be a relatively major release including:
+
+* Subtitles and captions.
+* A large rewrite of the facetting system.
+* Improved theme options.
+* Better stacking
+* [Numerous bug fixes and minor improvements][news.md].
+
+The majority of this work was carried out by [Thomas Pederson](https://github.com/thomasp85), who I was lucky to have as my "ggplot2 intern" this summer. Make sure to check out other visualisation packages: [ggraph](https://github.com/thomasp85/ggraph), [ggforce](https://github.com/thomasp85/ggforce), and [tweenr](https://github.com/thomasp85/tweenr).
+
+## Subtitles and captions
+
+Thanks to [Bob Rudis](https://rud.is), you can now add subtitles and captions:
+
+```{r}
+ggplot(mpg, aes(displ, hwy)) +
+  geom_point(aes(color = class)) +
+  geom_smooth(se = FALSE, method = "loess") +
+  labs(
+    title = "Fuel efficiency generally decreases with engine size",
+    subtitle = "Two seaters (sports cars) are an exception because of their light weight",
+    caption = "Data from fueleconomy.gov"
+  )
+```
+
+These are controlled by the theme settings `plot.subtitle` and `plot.caption`.
+
+The plot title is now aligned to the left by default. To return to the previous centering, use `theme(plot.title = element_text(hjust = 0.5))`.
+
+## Facets
+
+The facet and layout implementation has been moved to ggproto and received a large rewrite and refactoring. This will allow others to create their own facetting systems, as descrbied in the *Extending ggplot2* vignette. Along with the rewrite a number of features and improvements has been added, most notably:
+  
+  * Functions in facetting formulas, thanks to 
+    [Dan Ruderman](https://github.com/DanRuderman).
+    
+    ```{r}
+    ggplot(diamonds, aes(carat, price)) + 
+      geom_hex(bins = 20) + 
+      facet_wrap(~cut_number(depth, 9))
+    ```
+
+  * Axes were dropped when the panels in `facet_wrap()` did not completely 
+    fill the rectangle. Now, an axis is drawn underneath the hanging panels:
+    
+    ```{r}
+    ggplot(mpg, aes(displ, hwy)) + 
+      geom_point() + 
+      facet_wrap(~class)
+    ```
+
+  * It is now possible to set the position of the axes through the `position` 
+    argument in the scale constructor:
+    
+    ```{r}
+    ggplot(mpg, aes(displ, hwy)) + 
+      geom_point() + 
+      scale_x_continuous(position = "top") + 
+      scale_y_continuous(position = "right")
+    ```
+
+  * It is now possible to display a secondary axis that is a linear 
+    transformation of the primary axis through the `sec.axis` argument:
+    
+    ```{r}
+    ggplot(mpg, aes(displ, hwy)) + 
+      geom_point() + 
+      scale_y_continuous(
+        "miles / galloon", 
+        sec.axis = sec_axis(~ 235 / ., name = "L / 100km")
+      )
+    ```
+
+  * Strips can be placed on any side, and the placement with respect to axes
+    can be controlled with the `strip.placement` theme option.
+    
+    ```{r}
+    ggplot(mpg, aes(displ, hwy)) + 
+      geom_point() + 
+      facet_wrap(~ drv, strip.position = "bottom") + 
+      theme(
+        strip.placement = "outside",
+        strip.background = element_blank(),
+        strip.text = element_text(face = "bold")
+      ) +
+      xlab(NULL)
+    ```
+
+## Theming
+
+*   Blank elements can now be overridden again so you get the expected 
+    behavior when setting e.g. `axis.line.x`. Furthermore, `element_line()` 
+    gets an `arrow` argument that lets you put arrows on axes.
+    
+    ```{r}
+    arrow <- arrow(length = unit(0.4, "cm"), type = "closed")
+    
+    ggplot(mpg, aes(displ, hwy)) + 
+      geom_point() + 
+      theme_minimal() + 
+      theme(
+        axis.line = element_line(arrow = arrow)
+      )
+    ```
+
+*   Control of legend styling has been improved. The whole legend area can be 
+    aligned according to the plot area and a box can be drawn around all legends:
+    
+    ```{r}
+    ggplot(mpg, aes(displ, hwy, shape = drv, colour = class)) + 
+      geom_point() + 
+      theme(
+        legend.justification = "top", 
+        legend.box.margin = margin(3, 3, 3, 3, "mm"), 
+        legend.box.background = element_rect(colour = "grey50")
+      )
+    ```
+
+*   `panel.margin` and `legend.margin` have been renamed to `panel.spacing` 
+    and `legend.spacing` respectively as this better indicates their roles.
+    A new `legend.margin` has been actually controls the margin around each 
+    legend.
+
+*   When computing the height of titles ggplot2, now inclues the height of the
+    descenders (i.e. the bits `g` and `y` that hang underneath). This makes 
+    improves the margins around titles, particularly the y axis label. I have 
+    also very slightly increased the inner margins of axis titles, and removed 
+    the outer margins. 
+
+*   The default themes has been tweaked by 
+    [Jean-Olivier Irisson](http://www.obs-vlfr.fr/~irisson/) making them better
+    match `theme_grey()`.
+
+*   Lastly, the `theme()` function now has named arguments so autocomplete
+    and documentation suggestions are vastly improved.
+
+## Stacking bars
+  
+`position_stack()` and `position_fill()` now stacks values in the reverse 
+order of the grouping, which makes the default stack order match the legend.
+
+```{r}
+avg_price <- diamonds %>% 
+  group_by(cut, color) %>% 
+  summarise(price = mean(price)) %>% 
+  ungroup() %>% 
+  mutate(price_rel = price - mean(price))
+
+ggplot(avg_price) + 
+  geom_col(aes(x = cut, y = price, fill = color))
+```
+
+(Note also the new `geom_col()` which is short-hand for 
+`geom_bar(stat = "identity")`, contributed by Bob Rudis.)
+
+Furthermore, you can now stack negative values:
+
+```{r}
+ggplot(avg_price) + 
+  geom_col(aes(x = cut, y = price_rel, fill = color))
+```
+
+The overall ordering cannot necessarily be matched in the presence of 
+negative values, but the ordering on either side of the x-axis will match. 
+
+If you want to stack in the opposite, order try
+[`forcats::fct_rev()`](http://forcats.tidyverse.org/reference/fct_rev.html):
+
+```{r}
+ggplot(avg_price) + 
+  geom_col(aes(x = cut, y = price, fill = fct_rev(color)))
+```
+
+[news.md]: https://github.com/hadley/ggplot2/blob/master/NEWS.md