fdisc(x,D) := -D^2*0.5/x;
fcont(x,D) := -D/(1-D);
g(x,D) := if(fdisc(x,D)>=fcont(x,D)) then fcont(x,D) else fdisc(x,D);
my_x(t) := 0.5*t*(1-t);
my_y(t) := -t/(1-t);
load(draw);
draw2d(terminal=svg,file_name="Buck-boost_continuous_discontinuous",dimensions=[350,300],
  color=black,font="Times",proportional_axes=none,font_size=12,
  user_preamble="set termoption enhanced;set mxtics; set mytics; set key bottom right",
  xtics_axis=false,ytics_axis=false,grid=true,points_joined=true,yrange=[-6,1],
  xaxis=false,yaxis=true,axis_top=true,axis_bottom=true,axis_left=true,axis_right=true,
  line_width=2,xlabel="Normalized current",ylabel="Normalized voltage",
  label(["discontinuous",.075,-.6],["continuous",.175,-2.5]),
  label_alignment=right,nticks=50,
  label(["D=0.0",.24,.2],["D=0.2",.24,-.45],["D=0.4",.24,-.9],["D=0.6",.24,-1.7],
    ["D=0.8",.24,-4.2]),
  color=blue,key="",explicit(g(x,0.2),x,0,.25), explicit(g(x,0.4),x,0,.25),
  explicit(g(x,.6),x,0,.25),explicit(g(x,.8),x,0,.25),explicit(g(x,0),x,0,.25),
  color=red,line_width=.75,line_type=dots,
  parametric(my_x(t),my_y(t),t,0,.99)
 );