Help Lines point of origin in tikz

I use this code:

documentclass[12pt,a4paper,oneside]{book}

usepackage{tikz}
usetikzlibrary{positioning, decorations.text, calc}
usepackage{lipsum}

begin{document}
lipsum[1]

begin{figure}[!ht]
centering
 tikzset{declare function={f(x)=(-0.06*(x-2)+0.5)*(x-2)*(x-2);}}% applied math style
 foreach Z in {1,...,42} {pgfmathsetmacro{X}{Z/10}%
 pgfmathsetmacro{Y}{f(X)+0.9*rnd}%
 ifnumZ=1
  xdefLstOne{(X,Y)}%
  xdefLstTwo{"(X,Y)"}%
 else
  xdefLstOne{LstOne (X,Y)}%
  xdefLstTwo{LstTwo,"(X,Y)"}%
 fi}%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
begin{minipage}[b]{0.32textwidth}centering
begin{tikzpicture}
 begin{scope}[local bounding box=over]
 foreach Z in {1,...,40}
 {pgfmathsetmacro{Last}{{LstTwo}[Z-1]}
 pgfmathsetmacro{Current}{{LstTwo}[Z]}
 pgfmathsetmacro{Next}{{LstTwo}[Z+1]}
  edeftemp{noexpandpath ($0.6*Current+0.2*Last+0.2*Next$)   coordinate   (pZ);}
  temp
  ifnumZ=1
  xdefLstThree{(pZ)}
  else
  xdefLstThree{LstThree (pZ)}
  fi}
 foreach Z in {1,...,42}
 {pgfmathsetmacro{Coor}{{LstTwo}[Z-1]}
 fill Coor circle[radius=1pt];}
 draw[thick,blue] plot[smooth] coordinates LstThree;
 end{scope}
 draw[style=help lines,step=0.5cm] ([xshift=-3pt,yshift=-3pt]over.south west) grid ([xshift=3pt,yshift=3pt]over.north east);
 draw[gray,thin] ([xshift=-3pt,yshift=3pt]over.north west) rectangle  ([xshift=3pt,yshift=-3pt]over.south east);
 draw[stealth-stealth,thick] ([xshift=-3pt,yshift=3pt]over.north west) |- ([xshift=3pt,yshift=-3pt]over.south east);
end{tikzpicture}
caption{Overfitting}label{fig:overfitting}
end{minipage}hfil
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
begin{minipage}[b]{0.32textwidth}centering
begin{tikzpicture}
 begin{scope}[local bounding box=under]
 foreach Z in {1,...,42}
 {pgfmathsetmacro{Coor}{{LstTwo}[Z-1]}
 fill Coor circle[radius=1pt];}
 draw[thick,blue] (0.1,0.4) -- (4.2,2);
 end{scope}
 draw[style=help lines,step=0.5cm] ([xshift=-3pt,yshift=-3pt]under.south west) grid ([xshift=3pt,yshift=3pt]under.north east);
 draw[gray,thin] ([xshift=-3pt,yshift=3pt]under.north west) rectangle  ([xshift=3pt,yshift=-3pt]under.south east);
 draw[stealth-stealth,thick] ([xshift=-3pt,yshift=3pt]under.north west) |- ([xshift=3pt,yshift=-3pt]under.south east);
end{tikzpicture}
caption{Underfitting}label{fig:underfitting}
end{minipage}hfil
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
begin{minipage}[b]{0.32textwidth}centering
begin{tikzpicture}
 begin{scope}[local bounding box=good]
 foreach Z in {1,...,42}
 {pgfmathsetmacro{Coor}{{LstTwo}[Z-1]}
 fill Coor circle[radius=1pt];}
 draw[thick,blue] plot[smooth,domain=0.1:4.2,variable=x] (x,{f(x)+0.45});
 end{scope}
 draw[style=help lines,step=0.5cm] ([xshift=-3pt,yshift=-3pt]good.south west) grid ([xshift=3pt,yshift=3pt]good.north east);
 draw[gray,thin] ([xshift=-3pt,yshift=3pt]good.north west) rectangle  ([xshift=3pt,yshift=-3pt]good.south east);
 draw[stealth-stealth,thick] ([xshift=-3pt,yshift=3pt]good.north west) |- ([xshift=3pt,yshift=-3pt]good.south east);
end{tikzpicture}
caption{Balance}label{fig:balance}
end{minipage}
end{figure}

lipsum[1]
end{document}

This outputs the following plots:

The point of origin for the help lines is inside the plot. The left most column and the bottom most row are therefore not full which they should be.
Is there a way to set the point of origin of the help lines to align with the bottom left corner of the plot?

 draw[style=help lines,step=0.5cm,shift={($(over.south west) - (3pt, 3pt)$)}] ([xshift=-3pt,yshift=-3pt]over.south west) grid ([xshift=3pt,yshift=3pt]over.north east);

documentclass[12pt,a4paper,oneside]{book}
usepackage{pgfplots}
pgfplotsset{compat=1.17}
usetikzlibrary{calc,
                decorations.text, 
                positioning}
usepackage[skip=1ex, 
            font=footnotesize, labelfont=bf]{caption}
usepackage{lipsum}

begin{document}
lipsum[1]

    begin{figure}[!ht]
centering
pgfplotsset{width=linewidth, % style of diagrams
    axis lines=center,
    scale only axis,
    grid,
    axis line style={->, semithick},
    tick pos=upper,
    xmin=0, ymax=40,
    ymin=0, ymax=6,
    xticklabels={}, yticklabels={}
            }

begin{minipage}[b]{0.32textwidth}
    centering
    begin{tikzpicture}
begin{axis}[
    declare function = {f(t)=(-0.06*(t-2)+0.5)*(t-2)*(t-2);},
restrict y to domain = 0:6
            ]
addplot [scatter, only marks, domain=0:40] {5*rnd};
addplot [red, thick, domain=0:40] {f(x)};
end{axis}
    end{tikzpicture}
caption{Overfiting}
    label{fig:balance}
end{minipage}%
hfill
begin{minipage}[b]{0.32textwidth}
    centering
    begin{tikzpicture}
begin{axis}[
    declare function = {f(t)=0.1*(t + 2) +1;},
            ]
addplot [scatter, only marks, domain=0:40] {5*rnd};
addplot [red, thick, domain=0:40] {f(x)};
end{axis}
    end{tikzpicture}
caption{Overfitting}
    label{fig:balance}
end{minipage}%
hfill
begin{minipage}[b]{0.32textwidth}
    centering
    begin{tikzpicture}
begin{axis}[
    declare function = {f(t)=0.01*(t - 20)*(t - 20)+1;},
            ]
addplot [scatter, only marks, domain=0:40] {5*rnd};
addplot [red, thick, domain=0:40] {f(x)};
end{axis}
    end{tikzpicture}
caption{Balance}
    label{fig:balance}
end{minipage}

end{figure}

lipsum[1]
end{document}