Question: plot label problem

Hi,

I have a problem with plot using maple. I want to plot x_c agaist y_r 

However, i have no idea how to make at label that   y_r  . In other word, i want to make r under y and ignore dash (-) that between y and r

and the same idea for x_c.

could you please help me in this plot label problem?

 

 

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