r can fit armax model what is xreg used for how to use xreg function what is the purpose of xreg what is the purpose and how to use the xreg argument when fitting an armax model in r what is the purpose of xreg in r can an arima model be fit in R which is the purpose of the xreg argument? what is xreg in r why not use xreg in r what is xreg r what is the purpose of xreg in arima r fit ramax model what is the purpose of the argument xreg what is xreg in r what is the xreg argument used for in arima? r why does arima use xreg can you use arima model r what does the xreg mean in arima? which parameter to use to fit a model in r