I am trying to run the following Stata command:

scatter AUC Alpha xline(bestAlpha, lcolor(blue))

, where bestAlpha is a global macro storing the value of 0.0.

When I just execute the command:

scatter AUC Alpha

everything works fine, but when I try to add a vertical line by adding 'xline(bestAlpha, lcolor(blue))' the program would result in error, and I am not sure why.

Just to provide more detail, when I do:

list bestAlpha

Stata gives me the list of length 124 with a value (=0) assigned at index 1, and the rest of the entries are just missing values

when I do:

display bestAlpha

however, Stata gives out its correct value 0.

What should I do to fix my error? The error message is:

Invalid varlist

Thank you,