I am not clear about your use case. But the only way that I can think of at this moment is to have floating sheets similar to your bar charts just with condition= bar value. and use this floating bar as trigger for Alert.
This is not an efficient solution as bar chart height changes.
In below snap
Hidden Filters are floating sheets aligned with actual bars.
Perhaps you could share your efforts and sample workbook. Someone could suggest you a solution or better alternative.