import matplotlib.pyplot as plt
import matplotlib.ticker as ticker

x = [0,5,9,10,15]
y = [0,1,2,3,4]

tick_spacing = 1

fig, ax = plt.subplots(1,1)
ax.plot(x,y)
ax.set_xticks([0,1,2,5,8,9,10,11,20])
ax.xaxis.set_major_locator(ticker.MultipleLocator(tick_spacing))
plt.show()