#1: It works for the measure but not for the dimension because a line chart requires a dimension and a measure
When I chose the expression RangeSum(Above(GROSS_MARGIN_EXPRESSION,0,RowNo())) in the dimension field, I got the error message : Invalid Dimension. May be a line chart does not support the Cumulative formula for the Y and X axis.
#2 color coding works for a not calculated measure (Gross Profit), not for a calculated measure (Gross Margin per Liter)
For the first question, I am assuming that you are looking for something like this:
if((sum([Days In Stock]) / count([Vehicle ID])) < 35,rgb(146,208,80),rgb(255,0,0))
And for the second question make use of an expression similar to this:
if((sum([Current Stock Budget])-sum([Stock Value])>0),rgb(146,208,80),rgb(255,0,0))