Fix incorrect grid alignment for subreddit traffic stats.

While looking at subreddit traffic graphs (e.g. http://www.reddit.com/r/kdrama/about/traffic), I noticed that the x-axis grid is out of alighment, when interpreting the graph against the respective data table.

In the following example: http://jsbin.com/qequcove/9 
All graphs are out of order by one unit (hour, day, month) to the right. For instance, in the graph **uniques by month**, you would need to shift the x-axis cyclically to the left by one month.

I've noticed that the data fed to jQuery Flot is in **reversed order**. Since the graph style is **stepping lines**, and due to the nature that jQuery Flot may be interpreting data linearly and constructing graphs based on that order, the x-axis grid appears to be out of alignment. 

This can be simply fixed by constructing Flot data in the right order.

See the following for the correct grid alignment: http://jsbin.com/qequcove/10

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Also, `s.maxValue` should only be written whenever a datum is added.