I've seen trend lines drawn on log scales, this is linear scale. Which one's right? 

At this scale it doesn't really matter I think.
Not sure if i was totally clear: If Y is a log instead of linear scale, the second "dip" will be above the line, so no longer a part of it. If you mean the slope's too insignificant to make a difference, just change the time scale.
educated guess: depends on whether the trend is exponential or not.