Fix ordering option for the barChart.

[mhodorogea]

Fix the bug: ordering not working on barChart.

If you write in the barChart definition:

1. chart.ordering(function (d) { return d.data.value; });
   It will order the barChart xAxis by value increasing.
2. chart.ordering(function (d) { return -d.data.value; });
   It will order the barChart xAxis by value decreasing.
3. chart.ordering(function (d) { return d.data.key; });
   It will order the barChart xAxis by key, alphabetical order.

[gordonwoodhull]

Thanks! Please try the tests with grunt test; apparently you have missed the empty case.

Also, a test or two for this feature would be appreciated.

[mhodorogea]

Now the "grunt test" should work.

[mhodorogea]

Some tests have been added to check ordering on barChart.

[gordonwoodhull]

Thanks @mhodorogea!

I think this should fix #772

[gordonwoodhull]

This is not quite right because it only sorts the first stack. If there are multiple stacks, the latter stacks will get tacked on unsorted, and ruin the order. Probably we need to flatten first, then sort.

[gordonwoodhull]

Sigh, it turns out this was still not quite correct, because ordinarily the ordering function takes a regular group {key,value} pair, so the default ordering was not correct and was comparing undefined, which can crash crossfilter (see #909).

I am reverting flattenStacks and changing the definition of _ordinalXDomain to pluck the data from the flattened data and order that, and changing the tests to use ordering functions that work like the other ordering functions (no .data).