Cumulative Pending Cases

You can try, this should count the project that where pending during the month:

VAR miDate = MIN('Date'[Date]) // Start of the month in your graph 
VAR mxDate = MAX('Date'[Date])  // End of the month in your graph


RETURN
CALCULATE ( 
    DISTINCTCOUNT( Data[GUID] ), 
    FILTER(
       ALL('Data'), 
       // Projects that stared within the month
       (miDate <= 'Data'[STARTDATE] &&  'Data'[STARTDATE] <= mxDate)
       ||
       // Projects that ended within the month
        (miDate <= 'Data'[ENDDATE] &&  'Data'[ENDDATE] <= mxDate)
       ||
       // Projects that are pending the whole month
        ('Data'[STARTDATE] <= miDate  &&  mxDate <= 'Data'[ENDDATE])
       ||
       // Projects that are pending the during month
        (miDate <= 'Data'[STARTDATE]  &&  'Data'[ENDDATE] <= mxDate)
    ),
    'Data'[STATUS] IN {"P","V","F"},     // Filters on pending
    ALL('Date')  // Removes the date filter (not sure if you need to this)
)

And if you would like to get those that are pending on the end of the month:

VAR mxDate = MAX('Date'[Date])  // End of the month in your graph


RETURN
CALCULATE ( 
    DISTINCTCOUNT( Data[GUID] ), 
    FILTER(
       ALL('Data'), 
        ('Data'[STARTDATE] <= mxDate  && mxDate <= 'Data'[ENDDATE] ) 
    ),
    'Data'[STATUS] IN {"P","V","F"},     // Filters on pending
    ALL('Date')  // Removes the date filter (not sure if you need to this)
)

If it does not work I hope that it directs you to the right direction :-)

Regards,

Kristjan