Add sort_by query parameter to TX bar charts

The sort_by parameter was missing, which caused the data to be
displayed in the order it was returned from the data API.
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These changes have been checked by @cliffshepp.
Add sort_by parameter to high-volume bar charts

This uses the parameter added in 30c9f7fc786b5eec36d599c12593a31a9426d70a.
Merge pull request #549 from alphagov/fix-ordering-of-tx-bar-charts

Fix ordering of high-volume bar charts