matplotlib-users

[Matplotlib-users] How to draw an arrow using matplotlib

[Chris <Ch...@Fu...>]

Dear friends,

I just start to use matplotlib, which looks quite promising for me. I 
need to draw a couple of arrows in my 2D plot. Is there a simple way to 
get it work?

Any suggustions are welcome.

Best regards,
Shen

Re: [Matplotlib-users] How to draw an arrow using matplotlib

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

>>>>> "Chris" == Chris  <Ch...@Fu...> writes:

    Chris> Dear friends, I just start to use matplotlib, which looks
    Chris> quite promising for me. I need to draw a couple of arrows
    Chris> in my 2D plot. Is there a simple way to get it work?

    Chris> Any suggustions are welcome.

I recommend creating an arrow class, derived from matplotlib.artist.Artist, that
contains a matplotlib.lines.Line2D for the arrow stem and a
matplotlib.patches.RegularPolygon  with numVertices=3 for the arrow
head.  You can control the rotation of the arrowhead with the
orientation argument.  

Once you have this class so defined, you can add it instances of it to
the axes with ax.add_artist(arrow).

I'll be happy to help out with a prototype if you have trouble.  Take
a look at matplotlib.table.Cell, which John Gill wrote to support
tables.  You can use this as a simple model for how to write new
artists (things that draw into a figure) composed of other artists.

It would be nice to have a fancy arrow class, that supported text
labeling, at the base, along the stem and at the arrowhead.  You could
also consider a more sophisticated polygon other than a triangle for
the arrowhead.

Finally, if you needed to draw *a lot of arrows*, order of a thousand
or more (eg for direction fields), a
matplotlib.collections.PolygonCollection would be the way to go for
efficiency.

JDH

[Matplotlib-users] Re: How to draw an arrow using matplotlib

[Chris <bi...@Fu...>]

Thanks a lot for the very very detail reply. I can not find the time to 
do it by myself at the moment. I will come back to this issue again 
after 2 two weeks.

Best regards,
Chris

John Hunter wrote:
>>>>>>"Chris" == Chris  <Ch...@Fu...> writes:
> 
> 
>     Chris> Dear friends, I just start to use matplotlib, which looks
>     Chris> quite promising for me. I need to draw a couple of arrows
>     Chris> in my 2D plot. Is there a simple way to get it work?
> 
>     Chris> Any suggustions are welcome.
> 
> I recommend creating an arrow class, derived from matplotlib.artist.Artist, that
> contains a matplotlib.lines.Line2D for the arrow stem and a
> matplotlib.patches.RegularPolygon  with numVertices=3 for the arrow
> head.  You can control the rotation of the arrowhead with the
> orientation argument.  
> 
> Once you have this class so defined, you can add it instances of it to
> the axes with ax.add_artist(arrow).
> 
> I'll be happy to help out with a prototype if you have trouble.  Take
> a look at matplotlib.table.Cell, which John Gill wrote to support
> tables.  You can use this as a simple model for how to write new
> artists (things that draw into a figure) composed of other artists.
> 
> It would be nice to have a fancy arrow class, that supported text
> labeling, at the base, along the stem and at the arrowhead.  You could
> also consider a more sophisticated polygon other than a triangle for
> the arrowhead.
> 
> Finally, if you needed to draw *a lot of arrows*, order of a thousand
> or more (eg for direction fields), a
> matplotlib.collections.PolygonCollection would be the way to go for
> efficiency.
> 
> JDH
> 
> 
> -------------------------------------------------------
> This SF.Net email is sponsored by:
> Sybase ASE Linux Express Edition - download now for FREE
> LinuxWorld Reader's Choice Award Winner for best database on Linux.
> http://ads.osdn.com/?ad_id=5588&alloc_id=12065&op=click

[Matplotlib-users] Re: How to draw an arrow using matplotlib

[Chris <bi...@Fu...>]

I tried with what you suggested to make an Arrow class. To begin with, I 
only put an Line2D instance. I searched for table stuff in axes.py, 
matlab.py to make the arrow importable from matplotlib.matlab. Then I 
also try a very very simple demo and I expect to see a simple line. 
However, I can not see it. Could someone tell me what is the problem. 
Here is my very very rudimental codes.

-----------------------------------------------------------------
arrow.py

from artist import Artist
from lines import Line2D
from patches import RegularPolygon

class Arrow(Artist):
	"""
	An arrow with an triangular head .

	"""
	def __init__(self, xdata=None, ydata=None, *args, **kwargs):
		
		Artist.__init__(self)
		print xdata, ydata
		self._stem = Line2D(xdata, ydata, *args, **kwargs)
		
	def draw(self, renderer):
		self._stem.draw(renderer)

------------------------------------------------------------
in file axes.py
   in function cla add:
         self.arrows = []
---------------------------
     def add_arrow(self, a):
         'Add an arrow instance to the list of axes arrows'
         self._set_artist_props(a)
         self.arrows.append(a)

     def arrow(self, x, y, *args, **kwargs):
         a = Arrow(x,y, *args, **kwargs)
         self.add_arrow(a)
         return a
-------------------------------------------------------------
in file matlab.py
	add two tring for importing arrow
-------------------------------------------------------------
arrow_demo.py

from matplotlib.matlab import *

x = [0,1]
y = [0,1]
arrow(x,y, color='b',linestyle='-')
show()
-------------------------------------------------------------

Best regards,
Chris

John Hunter wrote:
> I recommend creating an arrow class, derived from matplotlib.artist.Artist, that
> contains a matplotlib.lines.Line2D for the arrow stem and a
> matplotlib.patches.RegularPolygon  with numVertices=3 for the arrow
> head.  You can control the rotation of the arrowhead with the
> orientation argument.  
> 
> Once you have this class so defined, you can add it instances of it to
> the axes with ax.add_artist(arrow).
> 
> I'll be happy to help out with a prototype if you have trouble.  Take
> a look at matplotlib.table.Cell, which John Gill wrote to support
> tables.  You can use this as a simple model for how to write new
> artists (things that draw into a figure) composed of other artists.
> 
> It would be nice to have a fancy arrow class, that supported text
> labeling, at the base, along the stem and at the arrowhead.  You could
> also consider a more sophisticated polygon other than a triangle for
> the arrowhead.
> 
> Finally, if you needed to draw *a lot of arrows*, order of a thousand
> or more (eg for direction fields), a
> matplotlib.collections.PolygonCollection would be the way to go for
> efficiency.
> 
> JDH
> 
> 
> -------------------------------------------------------
> This SF.Net email is sponsored by:
> Sybase ASE Linux Express Edition - download now for FREE
> LinuxWorld Reader's Choice Award Winner for best database on Linux.
> http://ads.osdn.com/?ad_id=5588&alloc_id=12065&op=click

Re: [Matplotlib-users] Re: How to draw an arrow using matplotlib

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

>>>>> "Chris" == Chris  <bi...@Fu...> writes:

    Chris> I tried with what you suggested to make an Arrow class. To
    Chris> begin with, I only put an Line2D instance. I searched for
    Chris> table stuff in axes.py, matlab.py to make the arrow
    Chris> importable from matplotlib.matlab. Then I also try a very
    Chris> very simple demo and I expect to see a simple
    Chris> line. However, I can not see it. Could someone tell me what
    Chris> is the problem. Here is my very very rudimental codes.

Nowhere in your code do you actually draw the arrows in self.arrows.
In Axes.draw you would need to do

    for arrow in self.arrows: 
        arrow.draw(renderer)

And this should work.

However, you can avoid all the overhead of add_arrow, and keeping a
list of arrows in self.arrows simply by doing


     def arrow(self, x, y, *args, **kwargs):
         a = Arrow(x,y, *args, **kwargs)
         self.add_artist(a)
                 ^^^
         return a

Ie, call add_artist which is a generic method for adding artists to
the axes.

Hope this helps,
JDH

[Matplotlib-users] Re: How to draw an arrow using matplotlib

[Chris <bi...@Fu...>]

Dear John,

I see your points. However, after I changed to using add_artist. This is 
still nothing on the canvas. I think that there is some problem with the 
draw function of my arrow class. I have no idea what else should be 
there. At the moment, only one call in my case as below.

	def draw(self, renderer):
		self._stem.draw(renderer)

Could u tell what else should be called?

Best regards,
Chris

John Hunter wrote:
>>>>>>"Chris" == Chris  <bi...@Fu...> writes:
> 
> 
>     Chris> I tried with what you suggested to make an Arrow class. To
>     Chris> begin with, I only put an Line2D instance. I searched for
>     Chris> table stuff in axes.py, matlab.py to make the arrow
>     Chris> importable from matplotlib.matlab. Then I also try a very
>     Chris> very simple demo and I expect to see a simple
>     Chris> line. However, I can not see it. Could someone tell me what
>     Chris> is the problem. Here is my very very rudimental codes.
> 
> Nowhere in your code do you actually draw the arrows in self.arrows.
> In Axes.draw you would need to do
> 
>     for arrow in self.arrows: 
>         arrow.draw(renderer)
> 
> And this should work.
> 
> However, you can avoid all the overhead of add_arrow, and keeping a
> list of arrows in self.arrows simply by doing
> 
> 
>      def arrow(self, x, y, *args, **kwargs):
>          a = Arrow(x,y, *args, **kwargs)
>          self.add_artist(a)
>                  ^^^
>          return a
> 
> Ie, call add_artist which is a generic method for adding artists to
> the axes.
> 
> Hope this helps,
> JDH
> 
> 
> 
> -------------------------------------------------------
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Re: [Matplotlib-users] Re: How to draw an arrow using matplotlib

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

>>>>> "Chris" == Chris  <bi...@Fu...> writes:

    Chris> Dear John, I see your points. However, after I changed to
    Chris> using add_artist. This is still nothing on the canvas. I
    Chris> think that there is some problem with the draw function of
    Chris> my arrow class. I have no idea what else should be
    Chris> there. At the moment, only one call in my case as below.

    Chris> 	def draw(self, renderer): self._stem.draw(renderer)

    Chris> Could u tell what else should be called?

Your drawing function looks fine.  Could this be a problem with axes
limits.  Make sure the x and y limits are set properly with the xlim,
ylim or axis commands.  The limits are normally autoscaled by calling
plot, but you are not using it so you have to take care that the
limits are set.  After you get a working prototype, I can help you
with the autoscaling part.

If this doesn't fix your problem, email me your matlab.py, axes.py and
test code as attachments off-list and I'll take a look.

JDH