This feature isn't currently supported in the Web Framework (or with Splunk controls generally). You could potentially use JQuery to reach into the wrapped HighCharts control in the DOM, but that will get messy quick. And fragile.
One thing you could try is using the Web Framework's D3Chart control which has a bar chart that is much more configurable. It's more complex to use, but you should be able to attach handlers to whatever you want.