documentclass[twoside]{book}

usepackage{ tikz, pgfplots}

linespread{1.25}

usepackage{lipsum}

pagestyle{empty}
usepackage{cancel}

begin{document}

section{LaTeX Spacing Issues}

lipsum[1]

%begin{example}
textbf{Main Example:}
several texts and equations here. It seems that if there is not enough space for the graph, then LaTeX automatically adjust the space between lines/graphs.

[y = x]
Another equation,
[y = x^2]

[
begin{tikzpicture}[x=1cm,y=0.07cm]
  defxmin{0}
  defxmax{10}
  defymin{0}
  defymax{120}

  % grid
draw[style=help lines, ystep=10, xstep=1] (xmin,ymin) grid (xmax,ymax);

  % axes
  draw[->] (xmin,ymin) -- coordinate (x axis mid) (xmax+.5,ymin) node[below] {$x$};
  draw[->] (xmin,ymin) -- coordinate (y axis mid) (xmin,ymax+5) node[left] {$y$};

  % xticks and yticks
  foreach x in { 1, 2, ..., 10}
    node at (x, ymin) [below] {x};
  foreach y in {20, 40, ..., 120}
    node at (xmin,y) [left] {y};t
    draw[scale=1,domain=0:10,smooth,variable=x,blue, thick] plot ({x},{12*x});

foreach Point in {(1,12), (2,24), (3,36), (4,48), (5,60), (6,72), (7, 84)}{
    node at Point {textbullet};
node[below=.5cm] at (x axis mid) {Number of visitors};
node[left=.8cm,rotate=90] at (y axis mid) {Money taken };
}
end{tikzpicture}
]
%end{example}

subsection*{New section}
lipsum[1]