Formatting labels in Y axis

Hi , qmestu 

According to your description, you want to bold the data label in conditional.According to my research and testing, there is currently no features in Power BI to set the font size according to the situation. At present, only all data labels can be set to bold in the bar chart.

Also I searched some way to use dax bold font, but this only works in HTML, can't fit into bar chart:
How to bold and underline text in DAX Measures - Smantin Data

For your needs, you can try to use the condition format for Color to distinguish.

Tips and tricks for formatting in reports - Power BI | Microsoft Learn

By another , you can also try to use the new feature "Create dynamic format strings for measures" to check if it can meet your need:

Create dynamic format strings for measures in Power BI Desktop - Power BI | Microsoft Learn

Thank you for your time and sharing, and thank you for your support and understanding of PowerBI! 

Best Regards,

Aniya Zhang

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