Rotation Curve - Not Flat - Astropy code or bad data?

[Lisa Struebing]

Hello, Undergraduate Newbie to Astropy here. 

I'm trying to plot rotation curves to support the theory for dark matter. 

I'm not getting flat curves and I am not sure if it is my radial_velocity data (I did a histogram) or my code for calculations. 

If someone has time to take a look at it and point me in a direction, it would be awesome. This is my first Astropy code, so it is not elegant, but I'm happy to send my data file and .ipynb file if anyone can assist. 

Thanks,

Strueb