Fix category barchart (fix #910)

Fix #910 (closed).

To sum up when the user selects "expenses" only in the selector, the category barchart still displays all the transactions (only the pie charts are filtered). The only thing it does is to revert the amount: -10 becomes 10 creating a bar going to the top instead of a bar going to the bottom.

The first commit really filters the barchart transactions too, like for the pie charts. The second removes the sign invertion which is actually confusing (idea of @valdroff).

Example of the second commit result: