change parametric plot setup programmatically

[hata]

Hello, I'm struggling to find a way to change Xmin, Xmax, Ymin, Ymax variables programmatically before invoking the PLOT view.
So, I want to prevent the HP PRIME from deciding about these min. and max. values, because I want to control these myself (from within an application I'm developing).
Note that I can change Tmin and Tmax (independent variable). But this is NOT my question.

...
Xmin := -5; // Xmin changed to -5
STARTAPP(1) // start plotting: changed Xmin discarded
...

A second, related question: can you programmatically force an aspect ratio of 1 when plotting (like 'menu -> zoom -> square' from the plot menu)

Thank you