C incluedes 1e-6*i so C will normally be complex. Y adds C to a calculation, so Y will normally be complex. Z has 1/Y so Z will normally be complex. I has something ./ Z, so I will normally be complex.
You then plot t vs I, so you are plotting a real value against a (usually) complex value. plot() requires that the coordinates to it be real, with the one exception that if you use plot(P) and P is complex, then real(P) will be plotted against imag(P); this exception never applies if you specify the x (or t) coordinate as you do in this plot() call.
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