Re: [Matplotlib-users] Novice question: Am I using pyplot.rgrids correctly?

[Andre' Walker-L. <wal...@gm...>]

Hi Bob,

> I am a matplotlib.pyplot novice, but I have looked through various FAQs to no avail.
> 
> I am plotting a polar graph with some negative values of r.  Everything goes well until I try to use rgrids().  Am I doing something wrong?
> 
> I attach some demo scripts and their outputs.
> 
> version1.py uses rgrid() but only has positive values of r.  Everything works the way I expect it to (see version1.png).
> 
> version2.py has negative values of r and the same call to rgrid() and misbehaves (see version2.png).
> 
> version3.py is the same as version2.py, but without the call to rgrids() (but with negative values of r).  Again everything is as expected.
> 
> What am I doing wrong in version2.py?

I am not sure why things work in version 3 and do not work in version 2.  A guess would be how matplotlib is covering up illegal values of your "r" coordinate.  Here I am only speaking from the math point of view, and not what is actually happening in matplotlib.  I tired finding info in the docs, but did not succeed in getting 

The coordinates "t" and "r" are angle and radius.  In polar coordinates, negative values of the radial coordinate "r" are not well defined, since in this coordinate system, you can not have negative values of a radius.

So my **guess** is that your problems are related to this ill definition of the radial coordinate.  And that sometimes matplotlib can cover it up for you, and other times it complains more loudly.


Regards,

Andre