Aligning graphs

Question

I am trying to align 4 graphs in a paper. Previous searching led my to the master/slave trick (see code below). Compiling the code as it is gives the desired result, but if I comment the "\hspace{-.35cm}" where I put a "% HERE !!!!!" comment, then the graphs are no more aligned. I obtained the .35cm by trial and error, but I would like a clean solution. Is there any way to do so?

\documentclass[12pt]{article}               

\usepackage[margin=1in,  a4paper, pdftex]{geometry}


\usepackage{pgfplots}

\tikzset{
    master/.style={
        execute at end picture={
            \coordinate (lower right) at (current bounding box.south east);
            \coordinate (upper left) at (current bounding box.north west);
        }
    },
    slave/.style={
        execute at end picture={
            \pgfresetboundingbox
            \path (upper left) rectangle (lower right);
        }
    }
}


\begin{document}

 \begin{figure}%Fig1
  \begin{center}

  FIGURE 

  \begin{tikzpicture}[master]
    \begin{axis}[
      width=2.1in, height=2in,
      ymin=122, ymax=150, ylabel={y label 1},
      name=plot1,
      title={First title}
    ]
    \addplot [color=blue, mark size=3.5pt, only marks, mark=square*, mark options={solid,draw=blue}]
    table[row sep=crcr]{
    0.25 127.29\\
    0.5 145.68\\
    };
    \end{axis}
    \begin{axis}[
      width=2.1in, height=2in,
      xmin=0, xmax=0.5, 
      ymin=166, ymax=190, ylabel={y label 2},
      at=(plot1.right of south east),
      anchor=left of south west,
      title={Second title},
      ]
    \addplot [color=blue, solid, dashed, ultra thick]
    table[row sep=crcr]{
    0.02 170\\
    0.5 180\\
    };
    \end{axis}
  \end{tikzpicture}%


  \title{\textit{Graph B}}

  \begin{tikzpicture}[slave]
    \begin{axis}[
      width=2.1in, height=2in,
      xmin=60, xmax=200, xlabel={x label},
      ymin=-0.08, ymax=0.04, name=plot1, 
      ylabel={y label 3},
      title={3rd title },
      title style={yshift=1ex}
    ]
    \addplot [
    color=blue, solid, ultra thick, forget plot, smooth]
    table[row sep=crcr]{
    60 -1e-5\\
    200 1e-4\\
    };
    \end{axis}

\hspace{-.35cm}     % HERE !!!!!

    \begin{axis}[
      width=2.1in, height=2in,
      xmin=60, xmax=200, xlabel={x label},
      ylabel={y label 4},
      at=(plot1.right of south east),
      anchor=left of south west,
      title={4th title}
    ]
    \addplot [color=blue, solid, ultra thick, forget plot, smooth]
    table[row sep=crcr]{
    60 -2.5e-5\\
    200 0.0001\\
    };
    \end{axis}
    \end{tikzpicture}
  \end{center}
\end{figure}%Fig1

\end{document}

Answer 1

One shouldn't use any \hspace and the like in a tikzpicture environment. There is a special library for situations like yours, groupplots. It can be used as follows.

\documentclass[12pt]{article}               
\usepackage[margin=1in,a4paper,pdftex]{geometry}
\usepackage{pgfplots}
\pgfplotsset{compat=1.16}
\usepgfplotslibrary{groupplots}
\begin{document}
 \begin{figure}%Fig1
  \begin{center} % <-you may want to replace this by \centering
   \begin{tikzpicture}
    \begin{groupplot}[group style={group size=2 by 2,horizontal sep=2cm,vertical
    sep=2.5cm},width=2.1in, height=2in]
     \nextgroupplot[ymin=122, ymax=150, ylabel={y label 1},
       title={First title}]
      \addplot [color=blue, mark size=3.5pt, only marks, mark=square*, mark options={solid,draw=blue}]
      table[row sep=crcr]{
      0.25 127.29\\
      0.5 145.68\\
      };
     \nextgroupplot[xmin=0, xmax=0.5, 
       ymin=166, ymax=190, ylabel={y label 2},
       title={Second title}]
      \addplot [color=blue, solid, dashed, ultra thick]
      table[row sep=crcr]{
      0.02 170\\
      0.5 180\\
      };
     \nextgroupplot[xmin=60, xmax=200, xlabel={x label},
       ymin=-0.08, ymax=0.04,
       ylabel={y label 3},
       title={3rd title}
     ]
     \addplot[color=blue, solid, ultra thick, forget plot, smooth]
     table[row sep=crcr]{
     60 -1e-5\\
     200 1e-4\\
     };
     \nextgroupplot[xmin=60, xmax=200, xlabel={x label},
       ylabel={y label 4},
       title={4th title}
     ]
     \addplot [color=blue, solid, ultra thick, forget plot, smooth]
     table[row sep=crcr]{
     60 -2.5e-5\\
     200 0.0001\\
     };
    \end{groupplot}
    \path  (group c1r1.south east) -- (group c2r2.north west)
    node[midway,font=\large\sffamily]{panel 2};
   \end{tikzpicture}
  \end{center}
 \end{figure}

\end{document}