fixes #811

fixes #710

The commit adds a first alpha version of the SmallMultiple chart configuration as requested in #500
Two main components are used to describe the small multiples:
- `<GroupBy />` is a generic component that allows the specification of a group by operation and sorting order. The `by` prop request a function with two arguments, the spec and the datum. It's called for each spec and for each data point available in the data array of a spec. It should return a unique value used to group the data points into multiple series.
- `<SmallMultiples />` component with two main optional props: `splitVertically` or `splitHorizontally` where you can specify the `id` of a `GroupBy` operator to render vertically (one below the other) or horizontally (one aside the other) the series defined by that group operation. A preliminary style configuration can be used to configure inner and outer padding in percentage for the vertical or horizontal charts.

close #500 

Co-authored-by: Nick Partridge <nick.ryan.partridge@gmail.com>

…nts (#896)

This commit fixes the rendering issue when using negative values in area charts. The correct approach
is to draw the area with a zero baseline (or dummy 1 baseline for log y scale).

This commits adds the ability to render orphan data points displayed on a line or area chart. An orphan data point is a datapoint that doesn't have a next or a previous data point to connect to.

fix #783

This commit will fix a regression that prevents the correct use of `useDefaultGroupDomain`.
It also includes a small feature that allows the consumer to specify a `groupId` on the same prop to allow merging the series domain with the domain of a different `groupId`.

Co-authored-by: nickofthyme <nick.ryan.partridge@gmail.com>

# [24.1.0](v24.0.0...v24.1.0) (2020-11-24)

### Bug Fixes

* **area_charts:** correctly represent baseline with negative data points ([#896](#896)) ([d1243f1](d1243f1))
* **legend:** legend sizes with ordinal data ([#867](#867)) ([7559e0d](7559e0d)), closes [#811](#811)
* render orphan data points on lines and areas ([#900](#900)) ([0be282b](0be282b)), closes [#783](#783)
* specs swaps correctly reflected in state ([#901](#901)) ([7fba882](7fba882))

### Features

* **legend:** allow legend text to be copyable ([#877](#877)) ([9cd3459](9cd3459)), closes [#710](#710)
* allow clearing series colors from memory ([#899](#899)) ([ab1af38](ab1af38))
* merge series domain with the domain of another group ([#912](#912)) ([325b013](325b013))
* small multiples for XY charts (alpha) ([#793](#793)) ([d288208](d288208)), closes [#500](#500) [#500](#500)