Make legendRenderer property public in order to be externally customizable

[nagykatalin]

Goals ⚽

The main idea behind this pull request is to make the legend renderer externally customizable and overridable.

Our customer's request was to detect when the user's clicks on a legend and to highlight corresponding line on the chart.

Implementation Details 🚧

By making public the legendRenderer property (similarly to the renderer property) we would like to use our own legend renderer class which stores the exact location of the labels and in this way we can detect if the user clicked on a label or not.

[codecov-io]

Codecov Report

Merging #3445 into 4.0.0 will decrease coverage by 0.22%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##            4.0.0   #3445      +/-   ##
=========================================
- Coverage   29.42%   29.2%   -0.23%     
=========================================
  Files         114     117       +3     
  Lines       12605   13301     +696     
=========================================
+ Hits         3709    3884     +175     
- Misses       8896    9417     +521

Impacted Files	Coverage Δ
Source/Charts/Charts/BarLineChartViewBase.swift	26.53% <100%> (-0.17%)	⬇️
Source/Charts/Charts/ChartViewBase.swift	24.77% <100%> (+1.69%)	⬆️
Source/Charts/Utils/ViewPortHandler.swift	26.84% <0%> (-7.26%)	⬇️
...ata/Implementations/Standard/PieChartDataSet.swift	35.71% <0%> (-5.96%)	⬇️
...plementations/Standard/LineRadarChartDataSet.swift	16.66% <0%> (-4.39%)	⬇️
...urce/Charts/Formatters/DefaultValueFormatter.swift	57.5% <0%> (-4.04%)	⬇️
Source/Charts/Renderers/YAxisRenderer.swift	56.85% <0%> (-2.65%)	⬇️
...ta/Implementations/Standard/LineChartDataSet.swift	30.76% <0%> (-1.89%)	⬇️
...a/Implementations/Standard/CombinedChartData.swift	58.56% <0%> (-1.81%)	⬇️
Source/Charts/Renderers/BarChartRenderer.swift	45% <0%> (-1.51%)	⬇️
... and 49 more

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[jjatie]

@nagykatalin I'm okay with this, however please target 4.0.0 instead of master.
@liuxuan30 Any comments?

[liuxuan30]

I'm ok too, giving the fact other major renderers are already public.

But I recommend be careful about the parameters, as LegendRenderer(viewPortHandler: _viewPortHandler, legend: _legend), viewPortHandler may not be available outside.

Oh I just saw 4.0 has made viewPortHandler readable, should be fine.

The remaining question is shall we change

    /// The legend object containing all data associated with the legend
    @objc open internal(set) lazy var legend = Legend()

to full open access for legend object? @jjatie

[pmairoldi]

I think legend still being readonly is fine.

[pmairoldi]

Thanks 👍

[liuxuan30]

good to have you back:)