[Matplotlib-users] two y axes via twinx and legends

[belinda thom]

Hi,

This mailing list is great---I've gotten a couple very useful replies  
from others in a very short time period. Thanks!

And now, onto my next question. I need to construct a two-y-axis  
plot. I've found some hints on how to do this on the mailing archive  
(Subject: secondary y-axis, Date: 9/28/05).

I've got the basics working, but have run into the same problem this  
prior post did: I want a legend that lists content from both the  
"left-hand-sided" plots and the "right-hand-sided" ones.

It appears only one axis or the others data can appear in a legend.

Is there anyway to merge the two axes into a single legend?

Also, the mail archives I'm viewing look terrible: line breaks aren't  
in the usual place, things are presented with >'s in them (which  
would make sense if the line breaks were preserved, b/c they  
correspond to pieces of prior email content). I'm using Safari 2.0.4  
w/Mac OS X 10.4.8. Do others see the same thing, and if the answer is  
yes, are folks satisfied with this state of affairs?

Thanks,

--b

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Re: [Matplotlib-users] passing 1D or 2D arrays to contour, pcolor, image, plot

[Eric Firing]

Below is the last (or nearly so) message of a thread from last summer. 
I have now implemented option 3 in svn, so:

If y is 2-D, plot(y) plots the columns of y against the row-index.

If x is 1-D and y is 2-D, plot(x,y) plots the columns of y against x. 
(In this case, x can also be 2-D if it is a single column.)

If x is 2-D and y is 1-D, plot(x,y) plots y against each successive 
column of x.  (Again, y can also be a single column.)

If x and y are both 2-D, plot(x,y) plots columns of y against the 
corresponding columns of x.  They must have the same number of columns.

All of this is consistent with Matlab, as far as I know.  Apart from 
this compatibility aspect, the design tradeoff is between the appeal of 
plotting rows, on the grounds that they correspond to C storage order, 
versus the appeal of plotting columns, on the grounds that one tends to 
think of columns in a table as the natural vectors to be plotted.  I 
don't think it makes much difference in efficiency; transposing is cheap 
in numpy.  It is possible that plotting non-contiguous values triggers 
an additional array copy somewhere in the chain of operations.  I have 
not tried to figure out whether it does, or what the time penalty would 
be if it does, but I strongly doubt it would be a noticeable fraction of 
the total plot generation time.

The changes are only very lightly tested so far, so please look for bugs.

Eric


Mark Bakker wrote:
> You are right, concerning your comment below.
> That will work just fine,
> Mark
> 
> On 7/13/06, *Eric Firing* <[EMAIL PROTECTED] 
> > wrote:
> 
> 
> 
> But why is this better than the following?
> 
> plot(Z[0,:], Z[1:,:])
> 
> The latter would accomplish the same, be completely consistent with
> option 4, be completely explicit and unambiguous, require no more typing
> than using a kwarg, require no extra logic in the plot code, and
> require
> no extra documentation for the plot command.
> 
> Eric
>  >
>  > As you said, there will be many more opinions,
>  >
>  > Mark
>  >
>  >
>  >
>  > To summarize, the options seem to be:
>  >
>  > 1) Leave plot argument parsing alone.
>  > 2) Accept an Nx2 array in place of a pair of arguments
> containing x
>  > and y.
>  >
>  > 3) Implement the Matlab model.
>  > 4) Implement the Matlab model, but taking rows instead of
> columns in an
>  > X or Y array that is 2-D.
>  >
>  > I am open to arguments, but my preference is the Matlab
> model.  I don't
>  > think that the difference in native array storage order
> matters much.
>  > It is more important to have the API at the plot method and
> function
>  > level match the way people think.
>  >
>  > Eric

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Re: [Matplotlib-users] more outdated/unhelpful documentation and installation issues on Mac OS X 10.4

[belinda thom]

Note: there is a type-o in the above email.

   "W/MacPorts, I've been able to get the TkAgg backend to work"

should be

   "W/MacPython, I've been able to get the TkAgg backend to work"



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Re: [Matplotlib-users] two y axes via twinx and legends

[belinda thom]

Looks like I've been able to help myself on this one. I'll post this  
here b/c others had asked about this in prior emails and I never saw  
an answer given.

You can make your own custom legend by keeping the return values from  
each plot command:

e.g.

l1 = plot()
twinx()
l2 = plot()

legend([l1,l2],['y1s tag','y2s tag'])

works like a charm.

HTH
--b

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