Dear Sridevi Grandhi,
Good day!!
I understand your requirement i.e., you would like to add a subtitle to the Excel charts similar to the chart title. AFAIK, there is no option in the Excel charts to insert the subtitles but as a workaround, you can use the Text Boxfeature in the Excel app to insert the box on the chart and enter the text.
However, since you require to perform on multiple charts, I believe you can meet your requirement using Macro VBA, so please try to post your concern in the VBA community. Reference: https://learn.microsoft.com/en-us/office/vba/articles/feedback-support
Thanks for understanding.
Regards,