There is a minor discrepancy in the behaviour of the linewidth kwarg between producing plots in the pyplot window and saving these plots into a file. According to the matplotlib documentation this kwarg accepts a float value. But I noticed that you can also give the value as a string and the plot works just fine. In other words, both of

plot(x,y,linewidth=1.5) and
plot(x,y,linewidth='1.5')

work and produce identical line plots of y vs. x. However, if the latter option of string valued linewidth is used, it is not possible to save the plot into ps or eps files. Instead the pyplot window gives the error message

"float argument required, not str"

without indicating where exactly the error occurred. This is inconsistent behaviour. You should either be able to use string valued linewidth kwargs when saving plots into ps or eps files or using the string values should be entirely forbidden here.