[Qgis-user] Docs about data based styles

[Antonio Vico]

Hi everyone,

Is there any doc about how to data based styles work (origins, positive 
X-Y axes and rotation sign)?

In qgis master I am testing a marker style composing a vector field 
marker (with X and Y attribute based on data) and a svg marker (with 
angle and offset based on data) but I get incorrect results (I want a 
vector field with a svg marker oriented at the end).

I tested vector field marker and svg offset (using the same values for 
X,Y attibutes in vector field and X,Y offset in svg) but svg are not 
coincident with end of vector field (it seem that possitive Y direction 
is inverse for field marker and svg; changing sign in Y coordinates for 
svg marker done good position for svg marker).

After this, I tested add angle to svg marker but now rotation in svg 
marker is correct but not displacement and get bad result. I tested 
different possibilities but I can't get good solution because I can't 
understand the beaviour for rotation and displacement in svg marker.

Thanks in advance.