One way to go here is to have a zero-bar indication. This is when the value of the bar is zero, the bar to have a specific look, for example, it can be 2 pixels tall, have a yellow background and be slightly transparent. This way, the user will be able to see and click the bar. This you can do via the DefaultVisualStyleSelector.
Another option is to allow the users to click on the bar labels. This way a user will not be able to click the bar itself, but will be able to click the corresponding label. You can get this dove via the label definitions of the series, its Template property and an attached property that will actually toggle the selection.
I have attached a small project demonstrating both of these approaches. If you have a question about how to do something else you have in mind, feel free to ask us again.
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