Align plots in pgfplots

Question

Can somebody help me? I try to align two plots horizontally in pgfplots! In the preamble I have:

MWE*

---

\documentclass[12pt]{article}
\usepackage{pgfplots,subcaption}
\pgfplotsset{ compat=1.7,
dacrip plot/.style={ % Set common styles that can be applied to the axis environment
    x=2cm, y=2cm,                   % Unit vector length
    axis lines=middle,              % Axis lines go through (0,0)
    xtick=\empty, ytick=\empty,     % No tick marks
    ymin=0, ymax=1.75,              % Y axis range
    enlargelimits=upper,            % Increase upper range a bit
    clip=false,                     % Allow labels to extend beyond the axis
    disabledatascaling,             % Allow to draw TikZ objects without having to use (axis cs:<x>,<y>)
    x label style={anchor=north},   % Alignment of axis labels
    y label style={anchor=east},
    yticklabel style={align=right}
}
}    
\begin{document}

\begin{figure}[!h]\centering
\begin{subfigure}[t]{0.48\textwidth} \centering
\begin{tikzpicture}[baseline]
\begin{axis}[dacrip plot, xlabel=$f$, ylabel=$U$]
       \addplot [blue, ultra thick] coordinates {(0,0) (1,1) (2,1) };
        \draw [dashed] (0,1) -- (1,1) node [pos=0, anchor=east] {$U_{max}$};
   \end{axis}
\end{tikzpicture}
\subcaption{Plot 1}\label{fig:ufMna}
\end{subfigure}
\begin{subfigure}[t]{0.48\textwidth} \centering

\begin{tikzpicture}[baseline]
\begin{axis}[dacrip plot, xlabel=$n$, ylabel=$M$ ]
       \addplot [blue, ultra thick] coordinates {(0,1) (1,1)} parabola [bend at end] (2,0.3);
   \end{axis};

\end{tikzpicture}
\subcaption{Plot 2}\label{fig:ufMnb}
\end{subfigure}
\caption{2 Plots}
\label{fig:ufMn}
\end{figure}
\end{document}

---

The problem is that the x-axis of Plot 2 is to short. Another problem is that the alignment of the x-axis is there, but the captions of the two plots aren't aligned!

---

***** Modified from OP's snippet

Answer 1

I cannot compile all your stuff, because of missing headers, but this should align the caption:

Adding a \vphantom{f] in your xlabel will add the missing vertical space to align the subcaptions:

\documentclass[12pt]{article}
\usepackage{pgfplots,subcaption}
\pgfplotsset{ compat=1.7,
dacrip plot/.style={ % Set common styles that can be applied to the axis environment
    x=2cm, y=2cm,                   % Unit vector length
    axis lines=middle,              % Axis lines go through (0,0)
    xtick=\empty, ytick=\empty,     % No tick marks
    ymin=0, ymax=1.75,              % Y axis range
    enlargelimits=upper,            % Increase upper range a bit
    clip=false,                     % Allow labels to extend beyond the axis
    disabledatascaling,             % Allow to draw TikZ objects without having to use (axis cs:<x>,<y>)
    x label style={anchor=north},   % Alignment of axis labels
    y label style={anchor=east},
    yticklabel style={align=right}
}
}    
\begin{document}

\begin{figure}[!h]\centering
\begin{subfigure}[t]{0.48\textwidth} \centering
\begin{tikzpicture}[baseline]
\begin{axis}[dacrip plot, xlabel=$f$, ylabel=$U$]
       \addplot [blue, ultra thick] coordinates {(0,0) (1,1) (2,1) };
        \draw [dashed] (0,1) -- (1,1) node [pos=0, anchor=east] {$U_{max}$};
   \end{axis}
\end{tikzpicture}
\subcaption{Plot 1}\label{fig:ufMna}
\end{subfigure}
\begin{subfigure}[t]{0.48\textwidth} \centering

\begin{tikzpicture}[baseline]
\begin{axis}[dacrip plot, xlabel={$n$\vphantom{f}}, ylabel=$M$ , xmax=2]
       \addplot [blue, ultra thick] coordinates {(0,1) (1,1)} parabola [bend at end] (2,0.3);
   \end{axis};

\end{tikzpicture}
\subcaption{Plot 2}\label{fig:ufMnb}
\end{subfigure}
\caption{2 Plots}
\label{fig:ufMn}
\end{figure}
\end{document}

I think that TeX will use the bounding box to position picture and caption, but in your case, your 2 pictures do not have the same size (here depth). Adding a \vphantom in the xlabel should add the required space to align the bottom of each graph.

Edit: for the second question, you could manually adjust the x-axis via the xmax key.