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This is because when you write r.plot(), the plot is performed in terms of the default chart on E, which is that of cylindrical coordinates in your case. Accordingly, the axes of the plot are $\partial/\partial \rho$, $\partial/\partial\phi$ and $\partial/\partial z$. I guess you'd rather want the plot to be performed in a Cartesian frame instead, with the axes $\partial/\partial x$, $\partial/\partial y$ and $\partial/\partial z$. You should then specify it by means of the keyword argument chart:

r.plot(chart=E.cartesian_coordinates()

For a better sampling, you may use

r.plot(chart=E.cartesian_coordinates(), number_values=7)

This is because when you write r.plot(), the plot is performed in terms of the default chart on E, which is that of cylindrical coordinates in your case. Accordingly, the axes of the plot are $\partial/\partial \rho$, $\partial/\partial\phi$ and $\partial/\partial z$. I guess you'd rather want the plot to be performed in a Cartesian frame instead, with the axes $\partial/\partial x$, $\partial/\partial y$ and $\partial/\partial z$. You should then specify it by means of the keyword argument chart:

r.plot(chart=E.cartesian_coordinates()
r.plot(chart=E.cartesian_coordinates())

For a better sampling, you may use

r.plot(chart=E.cartesian_coordinates(), number_values=7)