I recently had the need to put a single dot of a different color on a graph, and noted that stata doesn't have an option to allow you to place a single dot on a graph. You can put a line using xline and yline options (see below). I came up with the following work around:
local avgmpg : display r(mean)
gen avgmpg = `avgmpg'
local avgprice : display r(mean)
gen avgprice = `avgprice'
twoway (scatter mpg price) ///
(scatter avgmpg price if _n==1, mcolor(red) legend(off) ///
text(`avgmpg' `avgprice' "average"))
What I did was create the average price and mpg as varabiles, then drew my primary scatter plot and then overlaid a single dot. The restriction if _n==1 tells stata to only use observation number one. This prevents me from having _N (total number of observations in the data set) dots all right on top of each other, which will use up more resources. The text option just allows me to place text at a given location.
If I wanted to intersecting lines at the averages, I would have done
twoway (scatter mpg price, xline(`avgprice') yline(`avgmpg'))