setting y-axis limit 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 20 and 250.

Get current axis via plt.gca(), and then set its limits:

ax = plt.gca()
ax.set_xlim([xmin, xmax])
ax.set_ylim([ymin, ymax])