Your best bet for information on the CustomChart (a specific type of iChart where the data series details are configured per value column into a line, bar, marker, etc) is the online help documentation: iChart Data Series Details - Content Development - SAP Library
For a Pareto Chart there is no dedicated chart type, but between the various iChart type options for Bar, GroupBar, and Custom Chart. You can simply return your dataset sorted descending by count/number, so in effect you can have something like a 'Top 10' view, showing things like the most frequently occurring issues, or the largest total amounts for the given time period or associated unit.