Here's a reference talking about the different coordinate
> systems accessible in matplotlib:
> I think what we need is to set the coordinate transform to
> be in and absolute, instead of relative, coordinate
> system, or to build one ourselves. But I don't know
> enough about matplotlib's internals to know if this is
> right. Comments?
This is probably what you want to do. You want to define your arrow
in something like points, then do a rotation, and then apply one of
the transformation offsets to place your arrow at an x,y location.
Note there is a bug in some version of matplotlib in the affine code
which is fixes in SVN -- this arrow should have it's base at 0.5, 0.5
and be pointing NW and measure 2 inches from base to tip. The arrow
size is independent of zoom and figure window size, which may or may
not be desirable....
from matplotlib.transforms import Affine, Value, zero
from matplotlib.patches import Polygon
from pylab import figure, show, nx
fig = figure()
ax = fig.add_subplot(111, xlim=(0,1), ylim=(0,1), autoscale_on=False)
sx = 144/72.*fig.dpi.get() # 2 inches
sy = 144/72.*fig.dpi.get()
theta = 45*nx.pi/180.
# arrow drawn in pixels
trans = Affine(
verts = [
# offset in data coords
trans.set_offset((0.5, 0.5), ax.transData)
poly = Polygon(verts, transform=trans)