Dimension column based on page date slicer value - possible?

Anonymous You can create a measure that calculates the preceding month based on the selected [First of Month] value from the slicer. 

Create a measure for the selected [First of Month]:

SelectedFirstOfMonth = SELECTEDVALUE('Calendar'[First of Month])

Create a measure for the preceding [First of Month]:

PrecedingFirstOfMonth = EOMONTH([SelectedFirstOfMonth], -1) + 1

Create a measure to filter the data for the preceding month:

DAX
PrecedingMonthData =
CALCULATE(
SUM('YourTable'[YourDataColumn]),
'Calendar'[First of Month] = [PrecedingFirstOfMonth]
)

Use the SelectedFirstOfMonth measure to filter the data for the selected month:

DAX
SelectedMonthData =
CALCULATE(
SUM('YourTable'[YourDataColumn]),
'Calendar'[First of Month] = [SelectedFirstOfMonth]
)

For the first graph, use the SelectedMonthData measure.
For the second graph, use the PrecedingMonthData measure.