You should add commas:
plot1, = plt.plot(a,b)
plot2, = plt.plot(a,c)
The reason you need the commas is because plt.plot() returns a tuple of line objects, no matter how many are actually created from the command. Without the comma, “plot1” and “plot2” are tuples instead of line objects, making the later call to plt.legend() fail.
The comma implicitly unpacks the results so that instead of a tuple, “plot1” and “plot2” automatically become the first objects within the tuple, i.e. the line objects you actually want.
http://matplotlib.sourceforge.net/users/legend_guide.html#adjusting-the-order-of-legend-items