Calculation on Grouped Columns

Select the down arrow for your Value in the visual (SLA count?) and then select "Show as % of row total".  That will convert the visual to a % of False and True for each CreatedMonth.

First, you're making this too complicated.  You only need three measures:

     RowCount = Number of rows in the Incidents2017 table (assuming they each have either a True or False Overdue Status.  This value will get filtered by whatever categorical value you have in a column (Overdue Status for you and will result in two measures -one True and one False count).

     Total Sum of  RowCount = Calculate (RowCount, ALLSELECTED) This value is not filtered by a categorical vaule (ALLSELECTED) and results in the sum of RowCount (for False) and RowCount (for True).

     %Distribution = Divide(RowCount, Total Sum of RowCount, 0) This value becomes your percentage distribution.

Then set your graph up as 

     Axis: CreatedMonth

     Value: %Distribution (will filter based on what you put in legend)

     Legend: Overdue Status (True/False will result automatically if that's all that's in the Overdue Status column

Your graph will default to a 100% stacked bar as that's what you're asking it to do - show me the % distribution of overdue status by each month created.  The % True and % False always sums to 100% for each month.

For what you're trying to do, I would show the % as a tooltip and use RowCount as the Value. Then the graph will show correctly based on the count sum and % of each bar portion will be shown in the tooltip when you hover over it.

Hope this helps.