Graph a conditional range

In my spreadsheet, I have two data ranges, one to enter a monthly number and one to show the year to date amount. I use an IF formula to enter a number in the YTD column if the monthly number exists or a null "" if there is no monthly number. However, when I graph the YTD range, all the null values graph at zero.

Is there a way to select the entire data range to graph, and have cells containing a formula returning NULL not show on the graph?