Problem with Running total vizualisation

 Hi,

I am having a problem when graphically displaying my MTD running total sales.

I use the following Dax measure that I was able to come up with after reading the http://www.daxpatterns.com/cumulative-total article.

MTD Running Total Sales = 
IF (
    MIN ( 'Calendar'[Date] )
        <= CALCULATE (
            MAX ( Sales[Date] );
            FILTER ( ALL ( Sales ); RELATED ( 'Calendar'[MonthOffset] ) = 0 )
        );
    CALCULATE (
        [Total Sales];
        FILTER (
            FILTER ( ALL ( 'Calendar' ); 'Calendar'[MonthOffset] = 0 );
            'Calendar'[Date] <= MAX ( 'Calendar'[Date] )
        )
    )
)

The Total Sales measure is just the explict measure for my Sales table (= SUM(Sales[Sales]))

My Calendar table provides offsets allowing me to easily create measure relative to the current date.

When displaying the result of my MTD Running Total Sales in a table, everything looks just fine, the values start and stop whenever they are supposed to:

Now, when displaying the same information using a line chart, using the Calendar[Date] on the X-Axis, everything looks just fine:

But when replacing the Dates with WeekNumbers (also coming from the Calendar table), here's what I get:

So it is basically displaying the last value calulated by the measure for weeks not part of the scope of the calculation.

I am not sure where the problem stands as the table displays things correcty and so does the line chart only when using the dates.

Any help here would be greatly appreciated.

Thanks!