Calculate proportion of groups using particular item

[citylights]

I have a table structured as below. I would like to show in a table or chart the proportion of "Groups" using each item in the "Item" column. What's the best way to do this? thanks.

[Anonymous]

[Proportion] =
DIVIDE(
	DISTINCTCOUNT( T[Group] ),
	COUNTROWS( ALL( T[Group] ) )
)

[citylights]

thanks Daxer! Your solution worked perfectly for the example I gave. As it turns out though, I oversimplified my example. The table has the majority of its rows filtered out based on several different columns. So it's more like this the picture below. Any help would be appreciated. Thanks.