From the documentation for the subplot function: "subplot(m,n,p) divides the current figure into an m-by-n grid and creates axes in the position specified by p. MATLAB® numbers subplot positions by row. The first subplot is the first column of the first row, the second subplot is the second column of the first row, and so on. If axes exist in the specified position, then this command makes the axes the current axes."
So your second subplot(2, 1, 1) command makes that the current axes. It does not, as it sounds like you expected it to, create another figure with an axes on it. To do that you would need to call figure. Alternately if you want the sine and tangent curves to be on the same axes, call hold on before calling fplot the second time on the subplot(2, 1, 1) axes.
