How can one draw a simple circle with radius x on a pgfplots graph using tikz?

I am having trouble drawing a simple circle on a graph using tikz. When I try cases from the documentation or other examples from different questions, I get a diagonal ellipse. Here’s what I have so far.

documentclass[12pt]{extarticle}

usepackage{tikz}
usepackage{pgfplots}
pgfplotsset{compat=1.16}

begin{document}

begin{tikzpicture}[scale=0.8]
  begin{axis}[
    axis y line=center,
    axis x line=middle, 
    axis on top=false,
    xmin=-6.5, xmax=6.5,
    ymin=-6.5, ymax=6.5,
    grid, 
    xtick={-6,...,6},
    xticklabels={-6,...,6},
    ytick={-6,...,6},
    yticklabels={-6,...,6},
    ]
     
    draw (0,0) circle [radius=3];
    
  end{axis}
end{tikzpicture}

end{document}

I end up with a diagonal ellipse when I use radius like so:

Then I tried using the ellipse format where I changed draw (0,0) circle [radius=3]; to draw (0,0) circle (45pt and 45pt); As you can see from my result,

I get a circle, but it only seems to line up on the x-axis, and it overshoots the y-axis. Is the graph not actually square, or is it the formatting for the circle that’s creating problems?

usepackage{tikz}
usepackage{pgfplots}
pgfplotsset{compat=1.16}

begin{document}

begin{tikzpicture}[scale=0.8]
  begin{axis}[
    axis y line=center,
    axis x line=middle, 
    axis on top=false,
    axis equal image,
    xmin=-6.5, xmax=6.5,
    ymin=-6.5, ymax=6.5,
    grid, 
    xtick={-6,...,6},
    xticklabels={-6,...,6},
    ytick={-6,...,6},
    yticklabels={-6,...,6},
    ]   

    draw (1,3) circle[radius=37pt];
  end{axis}

end{tikzpicture}

end{document}