Adding a greek letter in Y-axis label using xpose.VPC

Dear All, I want to add a greek letter in Y-axis label of a VPC plot using xpose.VPC plotting function. I am able to do it with expression() in X-axis and main title. However, when I use it in Y-axis, it didn't work and gave me an error message "Error in ylb != "Default" : comparison is not allowed for expressions". Does anyone know how to add a greek letter in Y-axis label when using xpose.VPC function for a VPC plot? Thanks, Yulan This message is intended only for the confidential use of the intended recipient(s). If you have received this communication in error, please notify the sender by reply e-mail, and delete the original message and any attachments. Any unauthorized disclosure, copying, or distribution of this message (including the attachments), or the taking of any action based on it, is strictly prohibited.