Average over time vs. sliced average

You could but you'll need to get tricky.  I can think of a few ways to achieve this, so i'll offer one method and if that isn't suitable i can suggest other ideas.

You'll need a single measure that shows everything on the market, using an ALL or ALLEXCEPT depending on what filters you want to constrain this with (i.e. maybe a date range).

Next, you'll need a measure that does the same as the previous, just without the ALL/ALLEXCEPT.  Now, put both those measures onto your line graph.

One issue you might face however, is that doing this you are going to have a shared Y-Axis.  This means unless you are using a function that normalizes data in some way, you might not be able to get much value out of the smaller line.  For example "SUM" data will be more problematic compared to data such as % growth.

Hi swong,

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Is is possible for me to show two lines on a line graph, one for the overall marketplace average, and one for the individual account selected? 

Using ALL/ALLEXCEPT could achieve it as Anonymous has mentioned above.  The formulas below are for your reference. 

overall marketplace average =
CALCULATE (
    AVERAGE ( 'RevenueTable'[Revenue] ),
    ALLEXCEPT ( RevenueTable, 'RevenueTable'[Column_Name_that_you_used_for_Axis] )
)

individual account selected = SUM ( 'RevenueTable'[Revenue] )

If you still cannot figure out the measure to do it, you can just post your table structures with some sample/mock data and your expected result. So that we can better assist on this issue. :smileyhappy:

Regards