[visionegg] What exactly does anchor mean?

I am new to VisionEgg and was playing with the target.py demo. Instead of oscillating back and forth I wanted to make the target pivot back and forth like a windshield wiper or metronome. I thought all I would have to do was fix the position and vary the orientation parameter with the target "anchored" at one end, but my target always pivoted about the center. This made it clear how little I understood about the meaning of the parameters.
Could someone direct me to where the notion of "anchor" and other stimuli parameters are more fully explained? The function reference seems to give the defaults without actually saying much about them.
Is there a way built in visionegg to make the center of an object, for the purposes of computing transformation effects, something other than the geometric center? Or is this the type of thing that one typically writes their own controller function for to reposition a target in a way that counteracts the rotation? This is not something I need for an experiment right now, it is just to help me better understand the stimuli modules and VisionEgg. 

Thank you,

Britt Anderson
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