Finally got it fixed! Was able to submit a support ticket and my problem was resolved - two statements were the key. These are located in Pivot>Data Groupings>Values>General (and they were settings for "Value 0" as that was the only Y value I was tracking):
Value: =IF(ISEMPTY(SUM(Fields!WY_Portion_Enum)), 0, SUM(Fields!WY_Portion_Enum))
By specifying "ISEMPTY" for Y, you're sure to get a "0" return in spots with no data. By specifying "=FIRST(Col.Fields!Start_Date_Enum)" the full set of X values from the column for the current series is returned, not just the X values that correlate to the data points in the series. This causes the chart to fill out across the X axis and display all the data points.
The following expression was used for displaying only labels that are not equal to zero: =IF(SUM(Fields!WY_Portion_Enum, 0) > 0, SUM(Fields!WY_Portion_Enum, 0) ,"")
Attached is a screenshot of the appropriate configuration.
Nevron support to the rescue! Thanks again