API Changes

~plotnine.geoms.geom_smooth gained an extra parameter
legend_fill_ratio that control the area of the legend that is
filled to indicate confidence intervals. (32)
plotnine.ggplot.save gained an extra parameter verbose. It no
longer guesses when to print information and when not to.
plotnine.ggplot.draw gained an extra parameter return_ggplot.
If the minor_breaks parameter of scales is a callable, it now
expects one argument, the limits. Previously it accepted breaks
and limits.

New Features

Changed parameter settings for ~plotnine.stats.stat_smooth.
1. Default span=0.75 instead of 2/3
2. When using loess smoothing, the control parameter surface is
  only set to the value 'direct' if predictions will be made
  outside the data range.
Better control of scale limits. You can now specify individual
limits of a scale.
```
scale_y_continuous(limits=(0, None))
xlim(None, 100)
```
You can also use ~plotnine.scales.expand_limits
Low and high ~plotnine.scales.scale limits can now be expanded
separately with different factors multiplicative and additive
factors.
The layer parameter show_legend can now accept a dict for finer
grained control of which aesthetics to exclude in the legend.
Infinite values are removed before statistical computations stats
(40).

stats also gained new parameter na_rm, that controls whether
missing values are removed before statistical computations.
~plotnine.qplot can now use the name and a Pandas series to label
the scales of the aesthetics.
You can now put stuff to add to a ggplot object into a list and add
that that instead. No need to wrap the list around the internal
class Layers.
```
lst = [geom_point(), geom_line()]
g = ggplot(df, aes('x', 'y'))
print(g + lst)
```
Using a list allows you to bundle up objects. It can be convenient
when creating some complicated plots. See the Periodic Table
Example.

Fixed bug where facetting led to a reordering of the data. This
would manifest as a bug for geoms where order was important. (26)
Fix bug where facetting by a column whose name (eg. class) is a
python keyword resulted in an exception. (28)
Fix bug where y-axis scaling was calculated from the xlim
argument.
Fix bug where initialising geoms from stats, and positions from
geoms, when passed as classes (e.g. stat_smooth(geom=geom_point),
would fail.
Fixed bug in plotnine.ggplot.save where s