Align a caption with an image

Question

I have the following code:

\documentclass{article}
\usepackage{float}
\usepackage{siunitx}
\usepackage{graphicx}

    \begin{document}
        By plotting the attenuation in $ \si{\decibel s} $ we get the following graphs:
        \begin{itemize}
            \item \underline{For Butterworth:}
            \vspace{-0.1cm}
            \begin{figure}[H]
                \centering
                \includegraphics[width=0.9\textwidth]{sample_photo.png}
                \caption{Tabulated LPP normalized attenuation curve for Butterworth.}
            \end{figure}
            There are $ n $ number of zeros, in our case $ n=3 $ zeros of each kind. As we can see, the transmission zeros are in the infinity...
        \end{itemize}
    \end{document}

which produces this output:

As you can see, it seems that the caption and the image are left-aligned in comparison with the overall text. Could be a way to move both figure and caption to the right? Adding \hspace for the image worked but not for the caption.

Thank you.

Answer 1

Since your image doesn't have to float, you can obtain a correct centring with a measuredfigure environment, from threeparttable, nested in a simple center environment:

    \documentclass{article}
    \usepackage[utf8]{inputenc}
    \usepackage[showframe]{geometry}
    \usepackage{caption}%loat,
    \usepackage{threeparttable, floatrow}
    \usepackage[demo]{graphicx}
    \usepackage{siunitx}

    \begin{document}

    By plotting the attenuation in $ \si{\decibel s} $ we get the following graphs:
    \begin{itemize}
        \item \underline{For Butterworth:}
        \vspace{-0.1cm}
        \begin{center}
            \begin{measuredfigure}%
                        \includegraphics[width=0.9\linewidth]{sample_photo.png}
                        \captionof{figure}{Tabulated LPP normalized attenuation curve for Butterworth.}
            \end{measuredfigure}
        \end{center}
        There are $ n $ number of zeros, in our case $ n=3 $ zeros of each kind. As we can see, the transmission zeros are in the infinity...
    \end{itemize}

    \end{document} 

Answer 2

You simply need the image without a float, make the width of the image relative to the \linewidth and use \captionof of the capt-of package.

Showing margin edges and without paragraph indentation maybe the example is more clear (note: the image is intentionally flattened,just remove the height have it proportionally):

\documentclass{article}
\usepackage[showframe]{geometry}
\usepackage{capt-of,lipsum,parskip}
\usepackage{graphicx}
\begin{document}
\lipsum[2]
\begin{itemize}
\item \lipsum[3]
\item For Butterworth:\par
\includegraphics[width=\linewidth,height=1cm]{example-image-a}
\captionof{figure}{Tabulated LPP normalized attenuation curve for Butterworth.} 
\lipsum[4]                
\item \lipsum[5]
\end{itemize}
\lipsum[6]
\end{document}

If you want a smaller image centered, it is enough put a \hfil bebore of the image, or enclose with {\centering ... \par} or with \begin{center}...\end{center}.