Good way to add an interactive horizontal line annotation?


I would like to add an interactive feature to our application, where a user drags a horizontal line down from a tool above the chart and sets it at a certain y-value within the chart.  As the line is added, the colors of the bars above the dotted line dynamically change.  Is there a good way to go about this?

(The image above refers to “bar” plots, but in your library I think they will be called “column” charts.)