I'm just getting to grasp with plots and tikzpictures on latex. I can draw lines and functions now and manipulate them how i want, but i need to plot a circle.
\begin{tikzpicture}
\begin{axis}[
axis lines = left,
xlabel = $x$,
ylabel = {$y$},
]
\addplot [
domain=-4:4,
samples=10,
color=red,
]
{2*x+1};
\addlegendentry{$2x+1$}
\end{axis}
\end{tikzpicture}
This is what i'm using for the line, and i need to add a circle on there.
One way to graph a circle would be to graph the upper and lower portions separately. For the circle of radius 3 centered at the origin you can use:
\addplot [domain=-3:3, blue] {sqrt(9-x^2)};
\addplot [domain=-3:3, blue] {-sqrt(9-x^2)};
Need to use axis equal=true to ensure that the scale used on the x-axis is the same as that on the y-axis. Otherwise, the circle will look like an ellipse.
I shifted the placement of the legend by setting legend style.
You can also use parametric equations if you prefer:
\addplot[domain=-180:180, My Style, blue] ({3*cos(x)},{3*sin(x)});
where the domain is in degrees as that is how the pgfplots trignometric function arguments are expected.
\documentclass[border=2pt]{standalone}
\usepackage{pgfplots}
\tikzset{My Style/.style={samples=100, thick}}
\begin{document}
\begin{tikzpicture}
\begin{axis}[
axis lines = middle,
xlabel = $x$,
ylabel = {$y$},
axis equal=true,
legend style={
cells={anchor=west},
legend pos=north west
},
]
\addplot [domain=-4:4, My Style, red] {2*x+1};
\addlegendentry{$y=2x+1$}
\addplot [domain=-3:3, My Style, blue] {sqrt(9-x^2)};
\addplot [domain=-3:3, My Style, blue] {-sqrt(9-x^2)};
\addlegendentry{$x^2+y^2=9$}
\end{axis}
\end{tikzpicture}
\end{document}
\addplot[only marks,mark=o,red,samples at={...}] ...where the first...lists the position of your circle(s) and the second...is as above in your code. – Oct 16 '18 at 16:26