Indeed, you're right about optional arguments in C. So I guess the DLL I'm looking at (rmchart) wasn't written in C, but probably some kind of Basic). This is how the particular function I'm interested in is described:
nResult (LONG) = RMC_Draw2File(
ByVal nCtrlId (LONG),
ByRef sFileName (ASCIIZ),
Optional ByVal nWidth (LONG),
Optional ByVal nHeight (LONG),
Optional ByVal nJPGQualityLevel (LONG)
And judging by the description, the behavior I need happens when the optional args are not provided.