Highlight current month in yearly view

A bit hacky, but you could just make a measure that only displays in the current month, then put that measure on a combo chart with your actual series.

The measure you'd need looks like this:

CurrentMonth = 
IF(
	HASONEVALUE( 'Date'[YearMonth] )
	,IF(
		VALUES( 'Date'[YearMonth] ) 
			= LOOKUPVALUE(
				'Date'[YearMonth]
				,'Date'[Date]
				,TODAY()
			)
		,<expression for height>
	)
)

You could just put 1 as your height expression and use two y axes, but you'll end up with an extra axis on one side ranging from 0 to 1. If you want to plot them on the same y axis, then you'd need to define something that's larger than the values in your line series. I used something like the following:

CurrentMonth = 
IF(
	HASONEVALUE( 'Date'[YearMonth] )
	,IF(
		VALUES( 'Date'[YearMonth] ) 
			= LOOKUPVALUE(
				'Date'[YearMonth]
				,'Date'[Date]
				,TODAY()
			)
		,MAXX(
			ALL( 'Date'[YearMonth] )
			,CALCULATE( SUM( FakeData[Value] ) )
		) * 1.25
	)
)

The only difference is the MAXX() in there. It finds the largest monthly value of FakeData[Value] and then multiplies it by 1.25 This guarantees it's larger than my line series (which is just SUM( FakeData[Value] )), but still proportional, allowing it to fit well on the same y axis.

The measure is fairly simple. First we check if there's exactly one value for [YearMonth] in context (necessary for the next IF() function).

Then we evaluate VALUES( 'Date'[YearMonth] ) - this gives us all distinct values of that field in context, if there's only 1, then it is cast as a scalar value implicitly, allowing us to compare with the result of LOOKUPVALUE() for equality.

LOOKUPVALUE() just returns the value of [YearMonth] where [Date] = TODAY(). If the [YearMonth] in context is the same as today's, we return the MAXX() or whatever other expression you used there. If not, we return blank. If there's more than one [YearMonth] in context (like in a subtotal of any sort), the measure returns blank.

Here's a sample PBI report where I've implemented this: