# Adding an extra y tick, also adds an unnecessary one in the other part of the plot

I want to construct a normal distribution probability plot. The x-axis is for the x-values of my bins, and the y axis shows the cumulative probability on the tick labels, but the y-values are the corresponding normal distribution z-values. The format that my professor uses is: major grid with labels for the y-values seen on my plot, and a minor grid with no labels, for y-values of 2 to 9 and 91 to 98 (the commented out line in the code).

The label-less minor grid is implemented using extra y ticks. However, when I add a y-value as an extra y tick, two minor grid lines appear. Why do they appear and how do I get rid of them?

\documentclass[12pt,table]{report}
\usepackage{tikz,pgfplots}
\begin{document}
\begin{tikzpicture}[font=\tiny,scale=1.6]
\begin{axis}[
xtick pos=left,
xtick={3.68,3.84,4.00,4.16,4.32,4.48,4.64,4.80,4.96,5.12},
xticklabels={3.68,3.84,4.00,4.16,4.32,4.48,4.64,4.80,4.96,5.12},
xticklabel style={shift={(0,0 |- {axis description cs:0,-0.97})}},
ytick={-2.33,-1.28,-0.84,-0.52,-0.25,0.01,0.25,0.52,0.84,1.28,2.33},
yticklabels={1.0,10.0,20.0,30.0,40.0,50.0,60.0,70.0,80.0,90.0,99.0},
%   extra y ticks={-2.05,-1.88,-1.75,-1.64,-1.55,-1.48,-1.41,
-1.34,1.34,1.41,1.55,1.64,1.75,1.88,2.05},   % All the extra ticks that I want
extra y ticks={-2.05,-1.88,-1.75}, % best shows issue
extra y tick style={tick style={draw=none},grid=minor},
minor grid style={line width=.2pt,draw=red!120},
extra y tick labels={},
major grid style={line width=.2pt,draw=gray!120},
grid=both,
x axis line style={draw=none},
axis x line=middle,
axis y line=middle,
xmin=3.6,
xmax=5.27,
ymin=-2.5,
ymax=2.5,
domain=3.599:5.2,
]
\end{axis}
\end{tikzpicture}
\end{document}


For pgfplots, a minor grid is N automatically placed ticks/grid lines, placed between each of the major ticks. The major ticks are placed at the specified tick locations.

So I think you want grid=major for the extra ticks, and to set the major grid style for the extra ticks alone:

extra y tick style={
tick style={draw=none},
grid=major,
major grid style={line width=.2pt,draw=red!120}
},


Complete code:

\documentclass[12pt,table]{report}
\usepackage{pgfplots}
\begin{document}
\begin{tikzpicture}[font=\tiny,scale=1.6]
\begin{axis}[
xtick pos=left,
xtick={3.68,3.84,4.00,4.16,4.32,4.48,4.64,4.80,4.96,5.12},
xticklabels={3.68,3.84,4.00,4.16,4.32,4.48,4.64,4.80,4.96,5.12},
xticklabel style={shift={(0,0 |- {axis description cs:0,-0.97})}},
ytick={-2.33,-1.28,-0.84,-0.52,-0.25,0.01,0.25,0.52,0.84,1.28,2.33},
yticklabels={1.0,10.0,20.0,30.0,40.0,50.0,60.0,70.0,80.0,90.0,99.0},
extra y ticks={-2.05,-1.88,-1.75,-1.64,-1.55,-1.48,-1.41,
-1.34,1.34,1.41,1.55,1.64,1.75,1.88,2.05},   % All the extra ticks that I want
extra y tick style={tick style={draw=none},grid=major, major grid style={line width=.2pt,draw=red!120}},
extra y tick labels={},
major grid style={line width=.2pt,draw=gray!120},
grid=both,
x axis line style={draw=none},
axis x line=middle,
axis y line=middle,
xmin=3.6,
xmax=5.27,
ymin=-2.5,
ymax=2.5,
domain=3.599:5.2,
]
\end{axis}
\end{tikzpicture}
\end{document}