Add an option for x-axis sorting

The problem

Normally, x-axis sorting is just determined by Views' built-in sort criteria and that works well enough.
However, in the scenario also described by #2629018: Fix grouped display for "incomplete" result sets, when you having "holes" in your underlying data, the order of the x-axis can become arbitrary, with (currently) no way to fix this.

An example

Using the same example as in the other issue, but this time with a missing value for country A:

Country | Year | GPD
---------------------
A       | 2012 |  8 T$
A       | 2014 | 10 T$
B       | 2012 |  3 T$
B       | 2013 |  4 T$
B       | 2014 |  5 T$

This will lead to the following chart:

(In this simple example, sorting descending by Country name would actually fix this, but this stops working when both countries have missing values.)

Proposed solution

Add an option to sort the x-axis by a custom field.

This will probably only be necessary if the underlying data does have "holes" in it, but could also be a helpful shortcut for (inexperienced) users to easily apply sorting in other cases.