@ weba and bigjonyz :

I prefer the y-axis inverted too, and I have obtained this modifying the plugin's code:

• if you have sketchup opened, close it

• open the sketchup's "Plugins" folder

• open the file "FPSNav.rb" in the folder "FPSNav" with a text editor (like notepad++)

• find this line (about at row 343):

yRotationTransformation = Geom;;Transformation.rotation(@FPSNav_eye, horizontalAxis, radiansToRotate)

and change it like this:

yRotationTransformation = Geom;;Transformation.rotation(@FPSNav_eye, horizontalAxis, -radiansToRotate)

(the only difference is a minus before "radiansToRotate")

• save the file and then open sketchup: now the y-axis should be inverted