The following is copied from 'traffic statistics help', it's referring to individual traffic stats, but the same would be true for the dpw average traffic.
"Charted by Painting not by Day
Usually traffic statistics are presented as clicks per day. In DPW, your traffic is presented by painting and not by the day the clicks occurred. This can be confusing because there are dates on the horizontal axis, however these are simply the dates your paintings were published and not when the clicks occurred.
So, if you published a painting on July 3rd, then all of the clicks on that painting would be charted on July 3rd regardless of when the clicks occurred. While this approach doesn't show your overall trend in traffic over time, it does show you the relative popularity of your paintings.
It also means that the lines in the chart will tend to slope downward to the right since the older paintings to the left have had more time to attract traffic. This does not mean that your traffic is necessarily decreasing over time."