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The problem is that the gridlines are looking for something that is only a certain type of number, not a symbolic expression.

sage: plot(x,0,2,gridlines=[[1.4],[]])

works, for instance, as would

sage: plot(x,0,2,gridlines=[[sqrt(2).n()],[]])

That's what I'd recommend as a workaround for now.

The real problem is that the code

           else:
               vl=vline
               st=vgridstyle
           subplot.axvline(vl,**st)

is sending a symbolic expression to matplotlib as vl, which it cannot handle. But anything that has a real n() method should be legitimate input.

This is now #10980.