# Animates the visiblity of line objects by changing their layers. Useful for
# animating sequential appearance but does not alter the mesh geometry of the
# lines - doesn't read in their original data.
# This code is crude - it only checks that the objects are visible or not on
# the correct layer. It doesn't do any checks to make sure that the objects
# are ONLY in the correct layers, though this doesn't matter for us here.

import bpy

def animatelines(scene):

    frame = bpy.context.scene.frame_current

    for obj in bpy.data.objects:
        if 'Line_' in obj.name:
            index = int(obj.name.split('_')[1])
            
            # Let's put objects we want visible on layer 1
            if index <= frame:
                obj.layers[0] = True
            
            # Invisible objects should be on layer 2 but not layer 1
            if index > frame:
                obj.layers[1] = True
                obj.layers[0] = False
            
# Run the script every time the frame has changed        
bpy.app.handlers.frame_change_pre.append(animatelines)