Those interested in representing graphically the Relative Supply (RS) curve of a simple 2 x 2 Ricardian model of international trade in Mathematica will find that this task requires the use of Plot[] and Piecewise[]. Below you can find an example to illustrate this:

Plot[{Piecewise[{{0,x<=0},{1.5,0<x<1}, {2,x>= 1}}]},{x,0,12},Exclusions->None, PlotStyle->{Thickness[0.01]}, Axes->True,AxesOrigin->{0,0}]

Each portion of the curve is defined in Piecewise as {function, range}. So Piecewise[{15, x<=5}, {3x, x>5}] is for a function that takes the value 15 if x is smaller than or equal to 5 and the value 3x if x is greater than 5. Note also that in the example above I utilized several options of the command Plot[]. You can find more information about these options at https://reference.wolfram.com/language/tutorial/Options.html .