[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
> 
> 
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