This sets the ‘allowance’ (default 0.8) as a fraction of the space between adjacent trace lines (determined by the major Y-ticks).

ch.Set 'ia' 0.9

This may be set to a value >1 if you have a set of lines which move along very nearly parallel to one another. However this only works well with a plain inner axis as otherwise the ticks and labels collide.

See also ...
ch.Trace – trace charts
IStyle – inner-axis style