I'm assuming this is for the out-of-the-box Bar Chart visualization with Qlik Sense. Currently, the visualization allows for legends on Color by Dimension and Color by Measure. It is not available for expressions.
All visualizations are considered to be 'Extensions'. Extensions can be added to your deployment of Qlik Sense or even created by you to visualize data in various ways (ie. such as in a Map, or in a Sunburst chart, or a Gantt chart, etc..).
Here is a link to Branch, a project website that may have a bar chart that accomplishes the specific need or task that you are looking to have addressed (that or you create your own): http://branch.qlik.com/#!/project
To create your own, please refer to the online help: Extend ‒ Qlik Sense