And since this command is just a simple wrapper for the -twoway line- command, all other -twoway- options are available:
trendline gdp_percap if inlist(regnum,1,2), time(year) category(regnum) lpattern(dash)

-if- statements are also allowed with the command:
trendline gdp_percap if inlist(regnum,1,2), time(year) category(regnum)

You can also change the statistic (any statistic available in the -collapse- command):
trendline gdp_percap, time(year) category(regnum) statistic(median)

And you can plot by group:
trendline gdp_percap, time(year) category(regnum)

The simplest form of the command plots mean values over time for your entire dataset:
trendline gdp_percap, time(year)

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