Ask for a summary judgment after the state rests or after the officer testifies
Generally, in California traffic courts, and since the officer is typically the "only witness" for the state (ans there is typically no prosecutor)... it would be
"after the officer testifies" that in his/her opinion, the OP was straddling... So how would a "summary judgment" benefit the defendant in any way without him/her putting on a case, to either refute the officer's testimony, add to it or to simply offer his/her own opinion, s/he does not believe s/he was lane straddling as opposed to simply changing lanes?
In my opinion, lane straddling versus a simple (legal) lane change will depend on a number of different factors including but not limited to the flow of traffic, the officer's vantage point as well as (YES) the distance/time it took you to get completely out of one lane and completely into another (though there is no particular statutory distance that would deem any lane change as a violation or not). A good rule of thumb may be in whether another vehicle's normal movement (in either lane) would have been obstructed by the OP's movement!
Its purely a judgement call, and the officer will win this one nine times out often!