How plot methods?

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The problem is that x is a symbolic expression, and it does not have a eint method. So, instead of plotting a smybolic expression, you should try to plot a Python function:

sage: def eint(x): 
....:     return RR(x).eint()

Now, you can plot this function:

sage: plot(eint, [-1,1], frame=True)

Note that i had to transform x as an element of RR, because the plot function iterates over some elements of [-1,1] viewed as Pyhon floats which do not have a eint method either:

sage: float(1.1).eint()
AttributeError: 'float' object has no attribute 'eint'

You can see what happens if we omit to convert the float elements into RR:

sage: def eint_noconvert(x): 
....:     return x.eint() 

sage: plot(eint_noconvert, [-1,1], frame=True)
AttributeError: 'float' object has no attribute 'eint'