Average temperature data of Cambridge, England and Cambridge, Massachusetts graphing [fltk]

Bjarne Stroustrup “Programming Principles and Practice Using C++”
Chapter 15 Exercise 11
Using GUI library called FLTK (Fast Light Tool Kit, “full tick”).

Output:

[code language=”cpp”]
// Philipp Siedler
// Bjarne Stroustrup’s PP
// Chapter 15 Exercise 11

#define _USE_MATH_DEFINES
#include "Simple_window.h"
#include "Graph.h"
#include <cmath>

ostream& operator<<(ostream& out, Point& a) {
out << "(" << a.x << "," << a.y << ")";
return out;
}

// layout
constexpr int xmax = 600;
constexpr int ymax = 600;

constexpr int x_orig = xmax / 2;
constexpr int y_orig = ymax / 2;
Point orig(x_orig, y_orig);

constexpr int r_min = -10;
constexpr int r_max = 10;

constexpr int n_points = 400;

constexpr int x_scale = 2;
constexpr int y_scale = 2;

constexpr int xoffset = 100;
constexpr int yoffset = 100;

constexpr int xspace = 100;
constexpr int yspace = 100;

constexpr int xlength = xmax – xoffset – xspace;
constexpr int ylength = ymax – yoffset – yspace;

int main()
try
{
Point tl(100, 100);
Simple_window win(tl, xmax, ymax, "Function graphs");

// x axis
Axis x(Axis::x, Point(xoffset, ymax – yoffset), xlength, 12,
"month Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec");
x.set_color(Color::black);
x.label.move(-100, 0);
win.attach(x);

// y axis
Axis y(Axis::y, Point(yoffset, ymax – yoffset), ylength, 20, "temperature in 0 – 20 C");
y.set_color(Color::black);
win.attach(y);

// data
// Cambridge, England monthly normal weather 2016
vector<double> camb_eng_temp = {
3.7,
3.8,
6.1,
8.0,
11.4,
14.3,
16.8,
16.9,
14.1,
10.5,
6.5,
4.7
};

// Cambridge, Massachusetts monthly normal weather 2016
vector<double> camb_mas_temp = {
-1.9,
-0.9,
3.7,
8.9,
14.6,
19.8,
23.1,
22.2,
18.2,
12.7,
7.4,
0.9
};

// graph
Open_polyline cet; // Cambridge, England
Open_polyline cmt; // Cambridge, Massachusetts

for (int i = 0; i < 12; i++) {
cet.add(Point(xoffset + i * (xlength / 12), (yspace + ylength) – camb_eng_temp[i] * 20));
cmt.add(Point(xoffset + i * (xlength / 12), (yspace + ylength) – camb_mas_temp[i] * 20));
}

cet.set_color(Color::red);
win.attach(cet);

cmt.set_color(Color::blue);
win.attach(cmt);

win.wait_for_button();
keep_window_open();
}
catch (exception& e) {
cerr << "error: " << e.what() << ‘\n’;
keep_window_open();
return 1;
}

catch (…) {
cerr << "Unknown exception!\n";
keep_window_open();
return 2;
}
[/code]