import numpy as np
import matplotlib.pyplot as plt

evt = np.load('dimuon.npy')

xmin, xmax, xbinwidth = 2.6, 3.6, 0.01
vy,edges = np.histogram(evt, bins=100, range=(xmin,xmax))
vx = 0.5*(edges[1:]+edges[:-1])
vyerr = vy**0.5

fig = plt.figure(figsize=(6,6), dpi=80)
plt.plot([xmin,xmax],[0.,0.],c='black',lw=2)
plt.errorbar(vx, vy, yerr = vyerr, fmt = '.')
plt.grid()
plt.show()