Please read through sections 10.1 through 10.6 of the AxiDraw user guide, which discusses what the AxiDraw does (it draws the lines that you ask it to), how to preview how your artwork will plot, and discuss the various types of inputs that you might provide to it.
From what I can see, you are providing the AxiDraw with an outlined shape, and asking it to plot that outline.
What I think that you are asking about is how to convert from a scanned shape into a centerline that a pen can follow. As section 10.6 of the user guide discusses, that problem is called handwriting trajectory reconstruction; there is no “simply” about it. If you want the AxiDraw to plot the center line of text, you will need to (one way or another) actually provide that center line of the text to the machine.
For short elements of text, like signatures, the word “sincerely” and so forth the best way to capture handwriting is to capture it in real time, as discussed in section 10.5 of the user guide.