matplotlib-users

[Matplotlib-users] xzeroaxis

[Alan G I. <ai...@am...>]

I'm a newbie to Matplotlib but a longtime gnuplot user.
Using the matlab module, what is the right way to
set xzeroaxis
(using gnuplot terminology).  Using axis and hline seems
cumbersome, so I assume there is a better way to implement
this common need.

Thank you,
Alan Isaac

Re: [Matplotlib-users] xzeroaxis

[Stephen W. <ste...@cs...>]

On Mon, 2004-09-06 at 06:28, Alan G Isaac wrote:
> I'm a newbie to Matplotlib but a longtime gnuplot user.
> Using the matlab module, what is the right way to
> set xzeroaxis

MATLAB doesn't have anything similar, AFAIK, but the following will
work:

def xzeroaxis(form=3D'k'):	# draw a line at y=3D0 on the current plot
   v=3Daxis()
   plot(v[0:2],[0,0],form)

def yzeroaxis(form=3D'k'):
   v=3Daxis()
   plot([0,0],v[2:4],form)

Is this worth adding to the distribution?  I doubt it is general enough
to warrant.

John:  when I do the following:
v=3Darange(-5,5)
plot(v,v**3)
xzeroaxis()
yzeroaxis()

the vertical line at x=3D0 doesn't quite go to top and bottom of the
plot.  This is with 0.62.4.

--=20
Stephen Walton <ste...@cs...>
Dept. of Physics & Astronomy, CSU Northridge

Re: [Matplotlib-users] xzeroaxis

[John H. <jdh...@ac...>]

>>>>> "Stephen" == Stephen Walton <ste...@cs...> writes:

    Stephen> On Mon, 2004-09-06 at 06:28, Alan G Isaac wrote:
    >> I'm a newbie to Matplotlib but a longtime gnuplot user.  Using
    >> the matlab module, what is the right way to set xzeroaxis

    Stephen> MATLAB doesn't have anything similar, AFAIK, but the
    Stephen> following will work:

    Stephen> def xzeroaxis(form='k'): # draw a line at y=0 on the
    Stephen> current plot v=axis() plot(v[0:2],[0,0],form)

    Stephen> def yzeroaxis(form='k'): v=axis() plot([0,0],v[2:4],form)

    Stephen> Is this worth adding to the distribution?  I doubt it is
    Stephen> general enough to warrant.

    Stephen> John: when I do the following: v=arange(-5,5)
    Stephen> plot(v,v**3) xzeroaxis() yzeroaxis()

    Stephen> the vertical line at x=0 doesn't quite go to top and
    Stephen> bottom of the plot.  This is with 0.62.4.

Without testing, my guess is that the autoscale magic is getting in
your way.  Every plot command calls autoscale, which in the absence of
bugs will include the data but may exceed it.  Ie, if you plot data
with a x range from 0.1 to 9.9, it may set the xlim from 0--10.

Thus, although your idea of using the axis return values to get the
min, max is nice, it fails when the autoscale chooses a wider range
for nice ticking.  The solution I posted a few minutes ago in which
the span is plotted in *axes* coordinates rather than data coordinates
circumvents this problem.

Cheers!
JDH

Re: [Matplotlib-users] xzeroaxis

[John H. <jdh...@ac...>]

>>>>> "Alan" == Alan G Isaac <ai...@am...> writes:

    Alan> I'm a newbie to Matplotlib but a longtime gnuplot user.
    Alan> Using the matlab module, what is the right way to set
    Alan> xzeroaxis (using gnuplot terminology).  Using axis and hline
    Alan> seems cumbersome, so I assume there is a better way to
    Alan> implement this common need.

    Alan> Thank you, Alan Isaac

I've not used set xzeroaxis.  I googled around a bit and gathered that
this draws a horizontal line from xmin to xmax at y=0.  There is not
equivalent currently in matplotlib so there is no clean and easy way
to do this, but Fernando Perez and I talked a bit at scipy about
adding some gnuplot compatibility functions and this would fit under
that umbrella.  I'm interested in getting some feedback about whether
folks think this is a good idea.  The worry is that we dump too much
into the axes to support matlab, IDL and gnuplot users.

Here's how you should do this under the current matplotlib.  What you
want to do is plot a line where the y coord is in data units and equal
to zero, and the x coord is in axes units and spans the xrange.  The
line stretches from left to right regardless of the xlimits.

Each axes provides a data transform and an axes transform, so that you
can specify lines/text/etc in either coord system; axes coords are 0,0
for lower left and 1,1 for upper right.  But what you want to do is
mix the data and axes transforms for x and y.  The transforms module
provides a helper function for this.  The following code is freestyle
(untested) but should work 

    from matplotlib.transforms import blend_xy_sep_transform

    ax = gca()
    trans = blend_xy_sep_transform( ax.transAxes, ax.transData)
    plot([0,1], [0,0], transform=trans)

Try this out and see if it behaves as you like.  I can add a methods
for this for xzero and yzero if you and others feel this would be
useful.  Rather than reusing the gnuplot names, it might be better to
provide a method that always draws a line from xmin to xmax at a given
height regardless of the xlim, and do the same for a vertical line at
some x.

JDH

Re[2]: [Matplotlib-users] xzeroaxis

[Alan G I. <ai...@am...>]

On Mon, 06 Sep 2004, John Hunter apparently wrote:
> I've not used set xzeroaxis.  I googled around a bit and gathered that
> this draws a horizontal line from xmin to xmax at y=0.

Right.

>  Fernando Perez and I talked a bit at scipy about
> adding some gnuplot compatibility functions and this would fit under
> that umbrella.  I'm interested in getting some feedback about whether
> folks think this is a good idea.

I care about functionality much more than syntax.
This is a common need, so it shd be easy to do.

>     from matplotlib.transforms import blend_xy_sep_transform
>     ax = gca()
>     trans = blend_xy_sep_transform( ax.transAxes, ax.transData)
>     plot([0,1], [0,0], transform=trans)
> Try this out and see if it behaves as you like.

So far so good.

> Rather than reusing the gnuplot names, it might be better to
> provide a method that always draws a line from xmin to xmax at a given
> height regardless of the xlim, and do the same for a vertical line at
> some x.

I agree.  I actually need the additional functionality.

So while we're generalizing let me dream.  Focus on the case
of vertical lines.  The ideal functionality would allow me
to use 'vlines' with a special value to indicate ymin and
ymax.  Ideally this same 'special values' convention would
extend to 'fill' (or maybe 'bar'), since I often need event
shading.  Perhaps the obvious special value is None?

fwiw,
Alan

Re: [Matplotlib-users] xzeroaxis

[Gary <pa...@in...>]

Stephen Walton wrote:

>On Mon, 2004-09-06 at 06:28, Alan G Isaac wrote:
>  
>
>>I'm a newbie to Matplotlib but a longtime gnuplot user.
>>Using the matlab module, what is the right way to
>>set xzeroaxis
>>    
>>
>
>MATLAB doesn't have anything similar, AFAIK, but the following will
>work:
>
>def xzeroaxis(form='k'):	# draw a line at y=0 on the current plot
>   v=axis()
>   plot(v[0:2],[0,0],form)
>  
>
[...]

I've accomplished this feature via:
ax = axis()
set(gca().  hlines([0.],  ax[0],  ax[1])[0],  linewidth=1)

I don't remember why I did it this way instead of just plotting.  Maybe 
I thought is was more clever.
Does this method avoid auto-scaling? 

-g

Re: [Matplotlib-users] xzeroaxis

[John H. <jdh...@ac...>]

>>>>> "Gary" == Gary  <pa...@in...> writes:

    Gary> I've accomplished this feature via: ax = axis() set(gca().
    Gary> hlines([0.], ax[0], ax[1])[0], linewidth=1)

    Gary> I don't remember why I did it this way instead of just
    Gary> plotting.  Maybe I thought is was more clever.  Does this
    Gary> method avoid auto-scaling? -g

Yes it does, only because hlines doesn't call autoscale (I just looked
it up).  This is probably a bug, since I don't see why the plot and
hlines method should have different behavior.  It looks like an
oversight.

I think plotting the horizontal extent in axes coords is the proper
way to go.

JDH