[Python] Define the size of a grid on a plot using Matplotlib

I am plotting using Matplotlib in Python. I want create plot with grid, and here is an example from a plotting tutorial. In my plot range if the y axis is from 0 to 14 and if I use pylab.grid(True) then it makes a grid with the size of square of two, but I want the size to be 1. How can I force it?

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