You can only set the legend values from the X-series values in the current release (like the pie chart i sample 6).
I guess you want to add a list of string values? The only way to add that is to write to the ChartXml yourself. You can check the syntax by editing the sheet in Excel the way you want it, save it as a .xlsx document --> rename *.zip and extract.
Look at the chart.xml files in \xl\charts.
My test looked like this
<c:ptCount val="4" />
Not that easy, so if its an option for you use a range instead. I hope I can add something better in the future.