code: def raster(event_times_list, color='k'):
    
    ax = plt.gca()
    for ith, trial in enumerate(event_times_list):
      plt.vlines(trial, ith + .5, ith + 1.5, color=color)
    plt.ylim(.5, len(event_times_list) + .5)
    return ax
docs: Creates a raster from spike trains.
    
    @param event_times_list (array) matrix containing times in which a cell fired
    @param color (string) color of spike in raster
    
    @return ax (int) position of plot axes