Add legend entry without overwrite auto generated

Ho,

i have a Combined Chart with one Stacked Bar Chart and two Line Charts, and two Limit Lines, but in legend i only show Charts entry and not Limit Lines.


If i use a Custom Legend, i have an opposite situation:

```
let legend: Legend = graphView.legend
legend.enabled = true

let legendEntryA: LegendEntry = LegendEntry()
legendEntryA.label = "LIMITLINE1"
legendEntryA.formColor = UIColor.black

let legendEntryB: LegendEntry = LegendEntry()
legendEntryB.label = "LIMITLINE2"
legendEntryB.formColor = UIColor.black

var legendArray: [LegendEntry]! = []
legendArray.append(legendEntryA)
legendArray.append(legendEntryB)
legend.setCustom(entries: legendArray)
```


How can i have both legend entries? (Possibly without setting all entries with setCustom...)

Thanks a lot!

## Charts Environment

**Charts version/Branch/Commit Number:** 3.3.0
**Xcode version:** 10.3
**Swift version:** 5
**Platform(s) running Charts:** MacBook Pro
**macOS version running Xcode:** 10.14.6

Start with the Legend and setCustom(entries:) entry points shown in the issue, then trace how a combined chart generates entries for charts and limit lines. The work is done when automatically generated chart entries and manually added limit-line entries can appear together without replacing one another; no test file is named in the report.

Written by the indexing model from the issue text.