TIKZ PRactice

\usepackage{pgfplots}
\pgfplotsset{compat=1.16}
\begin{document}
 
\begin{tikzpicture}
    \begin{axis}[
        width=0.9\textwidth,
        height=5cm,
        grid=major,
        xlabel={time(s)},
        ylabel={measurement},
    ]
    \addplot[
        red, mark=square*,
        mark  options={fill=red!10!white}]
    coordinates {
    (0, 1.2) (1, 1.5) (2, 1.3) (3, 1.8) (4, 2.1) (5, 2.0)
    };
    \end{axis}
\end{tikzpicture}
\end{document}

\usepackage{tikz}
\usetikzlibrary{shapes, arrows.meta, positioning, calc}
\begin{document}
\begin{tikzpicture}
\draw[help lines] (0,-1) grid (40,20);
\draw[blue, thick] (3,2) .. controls (4,1) and (5, 1.5) .. (6, 0.5);
\draw[green!10!black]  (0,-1) circle [radius=0.4cm];
\end{tikzpicture}
\end{document}