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I have 6 graphs and in I put 2 graphs in a row (Parallel) it means used 3 rows for 6 graphs. Now It having more spaces vertically (can see in image in RED) and horizontal (can see in the image in Blue).

Please help me, how can I reduce the size between subfigures horizontally and vertically?

begin{figure}[ht]

centring 

begin{subfigure}[b]{0.45textwidth}

centring 

includegraphics[scale=0.45]{RHKNN.eps}

caption{}

label{fig:a1}

end{subfigure}

hfill

centring 

begin{subfigure}[b]{0.45textwidth}

centring 

includegraphics[scale=0.45]{LHKNN.eps}

caption{}

label{fig:b1}

end{subfigure}

newline

%parmidskip % force a bit of vertical whitespace

%centring 

begin{subfigure}[b]{0.45textwidth}

  centring 

     includegraphics[scale=0.45]{TPKNN.eps}

     caption{}

     label{fig:c1}

end{subfigure}

hfill

centring 

begin{subfigure}[b]{0.45textwidth}

  centring 

     includegraphics[scale=0.45]{RHDT.eps}

     caption{}

     label{fig:d1}

end{subfigure}\

%parmidskip % force a bit of vertical whitespace

centring 

begin{subfigure}[b]{0.45textwidth}

  centring 

     includegraphics[scale=0.45]{LHDT.eps}

     caption{}

     label{fig:e1}

end{subfigure}

hfill

centring 

begin{subfigure}[b]{0.45textwidth}

  centring 

     includegraphics[scale=0.45]{TPDT.eps}

     caption{}

     label{fig:f1}

end{subfigure}

hfill

caption{The impact of long ABC; ~ref{fig:a1} a by using K; ~ref{fig:b1} H position  by using K; ~ref{fig:c1} f by using K; ~ref{fig:d1} z by using D; ~ref{fig:e1} a by using D; ~ref{fig:f1} T position by using D}

label{fig 4:gr2}

end{figure} 

After struggling 10 to 12 hours I reached some solution, I removed hfill from the code and used begin{figure*}[t!] instead of begin{figure}[ht].

Here is the code:

begin{figure*}[t!]

centring 

begin{subfigure}[b]{0.45textwidth}

centring 

includegraphics[scale=0.45]{RHKNN.eps}

caption{}

label{fig:a1}

end{subfigure}

centring 

begin{subfigure}[b]{0.45textwidth}

centring 

includegraphics[scale=0.45]{LHKNN.eps}

caption{}

label{fig:b1}

end{subfigure}

begin{subfigure}[b]{0.45textwidth}

  centring 

     includegraphics[scale=0.45]{TPKNN.eps}

     caption{}

     label{fig:c1}

end{subfigure}

centring 

begin{subfigure}[b]{0.45textwidth}

  centring 

     includegraphics[scale=0.45]{RHDT.eps}

     caption{}

     label{fig:d1}

end{subfigure}\

centring 

begin{subfigure}[b]{0.45textwidth}

  centring 

     includegraphics[scale=0.45]{LHDT.eps}

     caption{}

     label{fig:e1}

end{subfigure}

centring 

begin{subfigure}[b]{0.45textwidth}

  centring 

     includegraphics[scale=0.45]{TPDT.eps}

     caption{}

     label{fig:f1}

end{subfigure}



caption[A set of six sub figures.]{The impact of ABC:

textbf{(subref{fig:a1})} app by using K;

textbf{(subref{fig:b1})} La  by using Ka;

textbf{(subref{fig:c1})} Ta  by using K;

textbf{(subref{fig:d1})} Ra by using Da; 

textbf{(subref{fig:e1})} Aa by using Db;

textbf{(subref{fig:f1})} To by using Da

}%

label{fig 4:gr2}

end{figure*}