* fix #1830. credit from #2049

* add combined chart unit tests for iOS, tvOS (macOS only have build process)

* use iterater rather than index

Fixed using wrong axis (Issue #2257)

* Removed redundant ivars in favour of proper access control

* Moved initialization of axes to their declaration to keep the same optionality exposed.

* Replaced relevant `ChartUtils` methods with `Double` extensions

Improves readability.
`nextUp` is built in and provides the same functionality.

* Updated `ChartUtilsTests` to match changes

* add option to build demo projects unit tests on iOS

* add ChartsDemo-OSX build test.

fix a small bug

* Refactored ChartUtils method into CGPoint extension

* Replaced ChartUtils.defaultValueFormatter()

* Codestyle fixes

* Minor cleanup to Highlighter types

* Fixes for PR

* Pulled master and updated code style

* added DataApproximator+N extension

* fixed PR notes

Much nicer call sites.
Renamed some parameter names.
Removed `NSAttributedStringKey` where type inference was sufficient.
Minor tidy of drawText calls in AxisRenderers