DataFrame.dropna() fill Data and interpolating. Why?
As we can see, the dropna() Method drop rows with value and add value from NaN rows. I don't undestood why.

between  1990-04 and 1990-08 u can see that we have value. But the dropna Method drop theses rows, and they are float64 value. And the dropna() method fill 1990-09 and 1990-10 with value that didn't exist.

![image](https://user-images.githubusercontent.com/25715921/52920577-ba40f280-32ec-11e9-8f09-2eb89553d3ba.png)


When I plot de graphic, The original Looks Like:
![image](https://user-images.githubusercontent.com/25715921/52920801-84513d80-32ef-11e9-89cb-ee2e38cab074.png)

but when I use dropna:

![image](https://user-images.githubusercontent.com/25715921/52920882-35f06e80-32f0-11e9-9118-6bc7bd1b0e7f.png)


EDIT:
The output.show_versions:
![image](https://user-images.githubusercontent.com/25715921/53060095-fe242b00-3497-11e9-9564-1bc3fa33545c.png)
