Add Axis titles by SwiftUI "overlay" as a workaround.

* [x] I've read, understood, and done my best to follow the [*CONTRIBUTING guidelines](https://github.com/jjatie/Charts/blob/master/CONTRIBUTING.md).

## What did you do?

Try to add axis titles in SwiftUI. I could not find any builtin method or way with Charts to add axis labels. BTW, scientific charts MUST have axis titles or they will be rejected by any reviewer.

## What did you expect to happen?

A simple xAxis method to add a text as an axis title that describes what the axis values represent and their unit.

## What happened instead?

Nothing ;-)

Therefore, I used the [SwiftUI overlay function](https://developer.apple.com/documentation/swiftui/hstack/overlay(_:alignment:)) for Views to add my axes titles:

... see workaround below in section "Demo Project".

## Charts Environment

**Charts version/Branch/Commit Number: 3.5.0**
**Xcode version: 11.5**
**Swift version: 5**
**Platform(s) running Charts: iOS**
**macOS version running Xcode: 10.15.5**

## Demo Project

The "LinePlot()" structure returns a SwiftUI View with a single line graph, with variables for the x and y data as well as for the line color. The content of this view is actually irrelevant. Thanks to [Evgeny Basisty for his tutorial](https://medium.com/@zzzzbh/plotting-in-ios-using-charts-framework-with-swiftui-222034a2bea6) on how to use Charts with SwiftUI.

Now, by using _.padding_, _.offset_, and _alignment_, one can adjust the exact position of the axes titles.

My Code to "solve" this issue:

LinePlot(entries: populateXYCharData( xvec: myXData, yvec: myYData ) , mylinecolor: [UIColor(#colorLiteral(red: 0.5725490451, green: 0, blue: 0.2313725501, alpha: 1))] )
.padding(.top, 30 )
.padding(.leading, 30)
.overlay(Text("Amplitude A [a.u.]")
.font(.system(size: 15))
.fontWeight(.medium)
.rotationEffect(.degrees(270))
.offset(x: -60.0, y: 0.0),
alignment: .leading)
.overlay(Text("Frequency f [ Hz ]")
.font(.system(size: 15))
.fontWeight(.medium)
.offset(x: 0, y: 0),
alignment: .top)

Example of a frequency plot from an FFT analysis:

No source files, tests, or entry points are named. Start by locating the SwiftUI axis configuration and existing axis-label APIs, then define how x- and y-axis titles should be exposed and positioned without the overlay workaround. Done means a documented axis-title API that supports labels and units on both axes.

Written by the indexing model from the issue text.