The inner style sets the tickmark type as for normal axes, and may be used to force the range to include zero. Style ‘exact’ will prevent it from rounding up to the next whole tick interval.

chSet 'istyle' 'forcezero,xtick'

The ‘xtick’ style works well here when combined with the default ‘ntick’ on the outer Y-axis. You might also consider ‘ptick’ which will label the inner axis down the inside of the Y-axis, so keeping the outer and inner labels well separated. For qualitative data (common on Kite charts) style ‘Plain’ works best.

See also ...
chTrace – trace charts
Tick Mark Styles