US sales: X6 vs. Z4 vs. 6-series
 

Came across this chart showing sales of X6 vs. Z4 and 6-series.
The dark brown/gray line shows X6 sales, blue line shows Z4 sales and yellow line shows 6-series sales.

For the most part X6 sales are close to Z4 and 6-series, since the X6 entry in 2008. Some deviations due to Z4 and 6-series MY exit/entry. Also, Z4 sales are seasonal.

Source: BmwConfig.com | Historical sales of BMW vehicles

I think you mean the yellow is the 6 series and the "dark brown line" is actually gray.

Fixed. Thanks.