plot a histogram. For internal use only

def _histogram(self, which, mu, sigma, data):
        """plot a histogram. For internal use only"""

        weights = np.ones_like(data)/len(data) # make bar heights sum to 100%
        n, bins, patches = plt.hist(data, bins=25, weights=weights, facecolor='blue', alpha=0.5)

        plt.title(r'%s %s: $\mu=%.2f$, $\sigma=%.2f$' % (self.name, which.capitalize(), mu, sigma))
        plt.xlabel('Items' if which == 'count' else 'Seconds')
        plt.ylabel('Frequency')
        plt.gca().yaxis.set_major_formatter(FuncFormatter(lambda y, position: "{:.1f}%".format(y*100)))