Um = 1.8
Um = 2.5
Im = 1.5
w = 1
fi = 1.1

u(t) = Um * sin(w*t + fi)
i(t) = Im * sin(w*t)
p(t) = u(t) * i(t)

set xrange[-fi-0.3:8]
set xtics ("0" 0, "−φ" -fi, "π/2" pi/2, "π" pi, "3π/2" 3*pi/2, "2π" 2*pi, "5π/2" 5*pi/2)
set ytics ("Um" Um, "Im" Im, "0" 0, "−Um" -Um, "−Im" -Im)
set xzeroaxis
set yzeroaxis
set key bottom right

set style line 1 lc rgb 'red' lt 1
set style line 2 lc rgb 'dark-green' lt 1
set style line 3 lc rgb 'blue' lt 1
set terminal svg font "Arial,12"
set output 'AC power graph f1.1.svg'
set encoding utf8

plot u(x) t "u(t)" with lines linestyle 1, i(x) t "i(t)" with lines linestyle 2, p(x) t "p(t)" with lines linestyle 3

set output