Does bathtub shaped distribution represent non-stationary request pattern?
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I have the data of inter-arrival times of vehicles. When I plot the data, I get a graph similar to bathtub shape. 
![enter image description here][1]


  [1]: http://i.stack.imgur.com/kozQP.jpg

The figure shows the mean inter-arrival time per hour. In my work, inter-arrival time of a vehicle represents the inter-arrival time of a request. My question is "Can I call this request pattern as non-stationary?" The mean inter-arrival time changes each hour. 