Add customized sort column at time series chart

[UpTownCat]

I am sorry about previous pull request modified sql directly, I have digged the code and change the order column before it generates sql when chart is time series chart.

[mistercrunch]

I don't understand what this fixes. Can you explain the difference between the before/after?

[UpTownCat]

When I need to export time series chart to csv file, I found I could not sort by specified column from metrics. The default sort column is the first column at metrics, even if I selected another column.

So I change some code and accept the specified sort column from front end. After that, the default sort column is date column if user does not specified sort column, and user can specified sort column whatever he wants from metrics when he needs to export csv file