plotting exp signal in matlab
A snippet from the MATLAB documentation of the plot command:


  plot(Y) creates a 2-D line plot of the data in Y versus the index of each value...If Y is complex, then the plot function plots the imaginary part of Y versus the real part of Y, such that plot(Y) is equivalent to plot(real(Y), imag(Y)).


The circular plot you're seeing is corresponding to the real part on the x-axis and the imaginary part of the y-axis. 

If you're doing plot(s), then MATLAB is plotting each column in s.