Depends what you accept as "a general equation" and what is "a plot", for example under $l_1$ metric the equation of what is perceived as a circle ( points having the same distance to a center) ends up looking like a square. Now would that count as genuine plot? or are we only allowed the Eucliean metric for plotting? also is an aswer containing a limit accpetable? for example $$x^{2n}+y^{2n}=r^{2n} \lim_{n \to +\infty}$$ is a square ( or is it? ) see this for $n=18$

also is the equation implicit in x and y or polar and parametric plots are also allowed?

Depends what you accept as "a general equation" and what is "a plot". For example, under the $l_1$ metric, the equation of what is perceived as a circle ( points having the same distance to a center) ends up looking like a square. Now would that count as genuine plot? Or, are we only allowed the Euclidean metric for plotting? Also, is an answer containing a limit acceptable? For example $$x^{2n}+y^{2n}=r^{2n} \lim_{n \to +\infty}$$ is a square ( or is it? ) see this for $n=18$

Also is the equation implicit in x and y or polar and parametric plots are also allowed?