Support "[ ]" in "Series" property to render multiple points from single data object

[joepavitt]

Description

• "Series" now has a "JSON" type option
  • If selected, this hides the "y" property option
  • Instead, the y-value is read from the keys provided in the series' array, e.g. ["value1", "value2"] would render a point on two series for their respective data points.
• Ensured support remains for all existing chart data types and entry points. Have also tested the new functionality on Bar/Scatter/Line charts.
• Updated documentation (both inline NR help & standalone docs) to include clearer examples for using this functionality and the "Series" property in general.

Related Issue(s)

Closes #293

Checklist

• I have read the contribution guidelines
• Documentation has been updated