OptionalgapThe icons to draw, lowest band first — resolveIconSetIcons or getConditionalIconSet.
A null is a band that draws nothing; it keeps its slot so the row still reads as the set.
Optionalorder'ascending' draws the lowest band first, the way the editor's threshold rows read;
'descending' draws the highest first, the way Excel's gallery shows a set (up, side, down).
OptionalsizeGlyph size in px.
Gap between glyphs in px.