[Python] How to set the axis limits in Matplotlib?

I need help with setting the limits of y-axis on matplotlib. Here is the code that I tried, unsuccessfully.

import matplotlib.pyplot as plt

plt.figure(1, figsize = (8.5,11))
plt.suptitle('plot title')
ax = []
aPlot = plt.subplot(321, axisbg = 'w', title = "Year 1")
ax.append(aPlot)
plt.plot(paramValues,plotDataPrice[0], color = '#340B8C',
marker = 'o', ms = 5, mfc = '#EB1717')
plt.xticks(paramValues)
plt.ylabel('Average Price')
plt.xlabel('Mark-up')
plt.grid(True)
plt.ylim((25,250))


With the data I have for this plot, I get y-axis limits of 20 and 200. However, I want the limits to be 20 and 250.

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