Вот что вы ищете, я думаю plt.xticks(positions, labels)
import matplotlib.pyplot as plt
import numpy as np
import pandas as pd
fig, ax = plt.subplots(dpi=220)
### used this to generate and test a new plot
data = pd.DataFrame(np.array(
[np.arange(50),
np.arange(50)]
).T).rename(columns={0: 'value_1', 1:'p_1'})
print(data)
data.plot(kind='bar', y='p_1', ax=ax, color ='red')
data.plot(kind='bar', y='value_1', ax=ax, color ='blue')
ax.set_xlabel("Index values")
ax.set_ylabel("Value 1 / P_1")
### added new code here
ticks = range(0, 50, 5)
labels = ticks
plt.xticks(ticks, labels)
#ax.legend(["Value 1, P_1"])
plt.title('Line plots')