Bollinger Band Angle Indicator

I've been attempting to code an indicator that plots the angle of the upper bollinger band from the current value to 5 bars back (or however many bars back you set that to).  And also do the same thing for the lower bollinger band.  And plot them both in a new window relative to a zero line.  This seems straight forward but I don't know javascript well enough to figure this out.  Can someone help?