Given this is accumulation you wont have NullValues if there is any data for the preceding dimensions. I used the Background color in the expression and set it to white() based on my master calendar dimention of Month To Date This year. This way my line disappears for any days after the current day, blends in with white background. But I still show the line for the MTD_LY through the end of the month.
If anyone has better idea for full accumulation charts let me know. Setting it to transparent would be better. Not sure of the code to do this. ?
if(_MTD_TY <> 1,white())