Issue tapping on chart to display marker

* [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?

I have just added a CombinedChartView with a bar and line chart and the displaying of marker on tapping on it, no possibility to zoom and move on the graph. It is fixed on the view.

## What did you expect to happen?

I expected to can tap on each element (bar or line) of the chart to display a marker with associated informations, no matter where I tap on the element. If I tap on bar it is the bar informations which are display and if I tap on the line of the line chart I whould like to have point of line chart informations.
Also I hoped to be able to tap outside charts (on empty parts without line and bar) without having a displayed marker.

## What happened instead?

Often when I try to tap on a bar the marker display is the marker of the line which is further down.

An other problem occurs when I have some long bar and a line which is at the middle of it. When I tap in the lower part of the bar (below the line chart) only line’s marker is display, instead of displaying a marker associated the tapping bar or line.

It seems the tapping on the bar chart is possible only on the upper part of bar but I would like to can tapped all over the bar and all over the line chart to display the associated marker.

Also when I tap outside charts (line or bar), marker is displayed but it shouldn’t, normally nothing should be displayed.

## Charts Environment

**Charts version/Branch/Commit Number:** 3.6.0
**Xcode version:** Xcode 12.0.1
**Swift version:** Swift 5
**Platform(s) running Charts:** iOS
**macOS version running Xcode:** macOS Catalina 10.15.7

## Demo Project

I have tested the process on the combined chart of the demo project of the lib. I have just kept the bar and the line chart, remove the double tapping to zoom and add the displaying of the marker(radarMarker) on tapping on chart.