Align last subfigure on the left side

Question

I have 5 pictures and want to align them that three pictures are on the left side above each other and two on the right side. I have tried it like this

\begin{figure}
\centering
\begin{subfigure}[h]{.48\textwidth}
\centering
\includegraphics[width=0.9\textwidth]{pictures/deltaphi_09_21.png}
\caption{Measurement 1: 2:1 CO$_2$-Argon.}
\label{fig:delta phi 1}
\end{subfigure}
\begin{subfigure}[h]{.48\textwidth}
\centering
\includegraphics[width=0.9\textwidth]{pictures/deltaphi_10_11.png}
\caption{Measurement 2: 1:1 CO$_2$-Argon.}
\label{fig:delta phi 2}
\end{subfigure}
\\
\begin{subfigure}[h]{.48\textwidth}
\centering
\includegraphics[width=0.9\textwidth]{pictures/deltaphi_08_21.png}
\caption{Measurement 3: 2:1 CO$_2$-Argon.}
\label{fig:delta phi 3}
\end{subfigure}
\begin{subfigure}[h]{.48\textwidth}
\centering
\includegraphics[width=0.9\textwidth]{pictures/deltaphi_08_11.png}
\caption{Measurement 4: 1:1 CO$_2$-Argon.}
\label{fig:delta phi 4}
\end{subfigure}
\\
\begin{subfigure}[h]{.48\textwidth}
\includegraphics[width=0.9\textwidth, left]{pictures/deltaphi_11_21.png}
\caption{Measurement 5: 2:1 CO$_2$-Argon.}
\label{fig:delta phi 5}
\end{subfigure}
\caption{Phase shift of the different plasma measurements over their incident power.}
\label{fig:delta phi over P} 
\end{figure}

My problem is that the last picture is still in the center but I want it exactly under the two left pictures.

Answer 1

I would delete all \centering directives, set the widths of the individual graphs to 1\textwidth rather than 0.9\textwidth, insert \hfill directives between adjacent subfigure environments, and issue \medskip directives between the rows of subfigures. And, for good measure, do consider loading a LaTeX chemistry package, such as mhchem, and replacing all instances of CO$_2$ with \ce{CO2}. (I believe that "ce" is short for "chemical expression".)

\documentclass{article} % or some other suitable document class
\usepackage[demo]{graphicx} % remove 'demo' option in real document
\usepackage{subcaption} % for subfigure environment
\usepackage{mhchem}     % for \ce macro
\begin{document}

\begin{figure}
\captionsetup[subfigure]{skip=0.333\baselineskip} % place captions closer to graphs

\begin{subfigure}{.48\textwidth}
\includegraphics[width=1\textwidth]{pictures/deltaphi_09_21.png}
\caption{Measurement 1: 2:1 \ce{CO2}-Argon.}
\label{fig:delta phi 1}
\end{subfigure}\hfill % <-- new
\begin{subfigure}{.48\textwidth}
\includegraphics[width=1\textwidth]{pictures/deltaphi_10_11.png}
\caption{Measurement 2: 1:1 \ce{CO2}-Argon.}
\label{fig:delta phi 2}
\end{subfigure}

\medskip % <-- new
\begin{subfigure}{.48\textwidth}
\includegraphics[width=1\textwidth]{pictures/deltaphi_08_21.png}
\caption{Measurement 3: 2:1 \ce{CO2}-Argon.}
\label{fig:delta phi 3}
\end{subfigure}\hfill % <-- new
\begin{subfigure}{.48\textwidth}
\includegraphics[width=1\textwidth]{pictures/deltaphi_08_11.png}
\caption{Measurement 4: 1:1 \ce{CO2}-Argon.}
\label{fig:delta phi 4}
\end{subfigure}

\medskip
\begin{subfigure}{.48\textwidth}
\includegraphics[width=1\textwidth]{pictures/deltaphi_11_21.png}
\caption{Measurement 5: 2:1 \ce{CO2}-Argon.}
\label{fig:delta phi 5}
\end{subfigure}

\caption{Phase shift of the different plasma measurements over their incident power.}
\label{fig:delta phi over P} 

\end{figure}

\end{document}

Answer 2

Add a "fake image" in the form of \hspace{0.48\textwidth} after the final (5th) image.

\documentclass{article}

\usepackage[draft]{graphicx}
\usepackage{subcaption}

\begin{document}

\begin{figure}
  \centering
  \begin{subfigure}{.48\textwidth}
    \centering
    \includegraphics[width=0.9\textwidth]{pictures/deltaphi_09_21.png}
    \caption{Measurement 1: 2:1 CO$_2$-Argon.}
  \end{subfigure}
  \begin{subfigure}{.48\textwidth}
    \centering
    \includegraphics[width=0.9\textwidth]{pictures/deltaphi_10_11.png}
    \caption{Measurement 2: 1:1 CO$_2$-Argon.}
  \end{subfigure}

  \begin{subfigure}{.48\textwidth}
    \centering
    \includegraphics[width=0.9\textwidth]{pictures/deltaphi_08_21.png}
    \caption{Measurement 3: 2:1 CO$_2$-Argon.}
  \end{subfigure}
  \begin{subfigure}{.48\textwidth}
    \centering
    \includegraphics[width=0.9\textwidth]{pictures/deltaphi_08_11.png}
    \caption{Measurement 4: 1:1 CO$_2$-Argon.}
  \end{subfigure}

  \begin{subfigure}{.48\textwidth}
    \centering
    \includegraphics[width=0.9\textwidth]{pictures/deltaphi_11_21.png}
    \caption{Measurement 5: 2:1 CO$_2$-Argon.}
  \end{subfigure}
  \hspace{.48\textwidth}\mbox{}
  \caption{Phase shift of the different plasma measurements over their incident power.}
\end{figure}

\end{document}