Modifying maxVisibleCount/Icon visibility

## What did you do?

I am using a combined chart to display a line graph as well as a scatter plot to indicate specific events on the line graph. The data points used for the scatter plot have icons associated with each specific kind of event. The issue is that when you zoom out far enough (past the maxVisibleCount of 100) the icons are replaced with circles. Would be possible to add functionality to the chart so that the icons are always visible?

## Charts Environment

**Charts version/Branch/Commit Number: 3.1.1
**Xcode version: 9.4.1
**Swift version: 4.1
**Platform(s) running Charts: iOS 10.0+
**macOS version running Xcode: 10.13.5

The report concerns combined line and scatter charts in Charts 3.1.1 on iOS, with icons changing after zooming beyond maxVisibleCount=100. Start by reproducing that zoom level and tracing the maxVisibleCount and icon-rendering behavior; done means event icons remain visible when zoomed out instead of becoming circles.

Written by the indexing model from the issue text.