Bug fixing with one line, updating ChartViewBase.swift

[Eric0625]

fixing a bug: when clearing chart, the 'lastHighlighted' var isn't set to nil. This will cause user to tap twice to highlight the new value, if the new value's index is the same as the old lastHighlighted's.

[liuxuan30]

do you mean you called clear() first, and then highlight? What's the context for clearing? Have you tried just set the new chart data directly?
First time I just clear() today :)

[Eric0625]

I tried to highlight the first bar in my barChart, then I press a button to remove old data and fill in new ones, but I find out you need to tap twice to highlight the first bar of new data.

And I don't know how to "set the new chart data directly" without calling clear().

open func clear()
    {
        _data = nil
        _offsetsCalculated = false
        _indicesToHighlight.removeAll()
        setNeedsDisplay()
    }

Function clear() first sets _data to nil, IMO that sort of means "set the new chart data directly". Then it called _indicesToHighlight.removeAll(), which deletes all highlighted slices in the chart, so what's the meaning to keep the lastHighlighted variable? It's now representing a non-exist highlighted object.

[liuxuan30]

What I mean is just call chartView.data = newData to update your entire chart data without calling clear()

[Eric0625]

That's exactly what I did when using the chart for the first time: only set chartView.data with new data. Then the app behaved like this:

This was the initial status: I clicked the first bar

Then I hit the button on top of it to fill the chart with new data, but you can see the first bar of new data is automatically selected, which is definitely not what I want:

I've tried several times but it looks like that only to both call chartView.lastHighlighted=nil and _indicesToHighlight.removeAll() can you get the fully cleaned chart.

[liuxuan30]

I see. The lastHighlighted is reserved by default, which seems by design. I think some users will prefer keep it while some don't, so seems like we should have on/off switch, not just simply clear it.

If you can add a switch for it, I think I can manage to merge this PR, or I will do it later.

[Eric0625]

Ok, I'll try it when I have time

[liuxuan30]

looks good to me. we will try fix travis first, and later handle some simple PRs, thanks for you patience.

[codecov-io]

Codecov Report

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@@            Coverage Diff            @@
##             master    #2355   +/-   ##
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  Coverage          ?   19.66%           
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  Files             ?      112           
  Lines             ?    13713           
  Branches          ?        0           
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  Hits              ?     2696           
  Misses            ?    11017           
  Partials          ?        0

Impacted Files	Coverage Δ
Source/Charts/Charts/ChartViewBase.swift	24.28% <0%> (ø)

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