Resize Vertical Tick Label Height (XYStepChart)

It’s not clear how you are formatting the dates now, but setDateFormatOverride in DateAxis allows you to specify a suitable SimpleDateFormat. If not already available, you should be able to override getShortMonths() in DateFormatSymbols for the Roman numerals.

Addendum: For correct localization, it may be easier to do something like this:

DateAxis axis = (DateAxis) plot.getDomainAxis();
DateFormatSymbols dfs = DateFormatSymbols.getInstance(); // default locale
String[] roman = { ... };
dfs.setShortMonths(roman);
axis.setDateFormatOverride(new SimpleDateFormat("dd-MMM-yyyy", dfs));