结合数据坐标和像素坐标

Question

I'm trying to add annotations at specific positions on a matplotlib figure. I want to specify a position in data coordinates and then offset that by a distance in pixel (or point) coordinates. It seems like I should be able to say

offset = ScaledTranslation(-6/72, -6/72, self.figure.dpi_scale_trans)
self.axes.annotate(u'\u00d7', xy=(wavelength, intensity), xycoords=offset)

I would expect this to position the annotation six pixels lower and six pixels to the left of x = wavelength , y = intensity . Instead I get an exception:

Exception in thread Thread-7:
Traceback (most recent call last):
  [...]
  File "/Users/bdesham/Projects/New GUI/application/views/spectrum_view.py", line 117, in update_graph_display
    self.figure.canvas.draw()
  File "/Users/bdesham/Code/virtualenv/lib/python2.7/site-packages/matplotlib/backends/backend_wxagg.py", line 44, in draw
    FigureCanvasAgg.draw(self)
  File "/Users/bdesham/Code/virtualenv/lib/python2.7/site-packages/matplotlib/backends/backend_agg.py", line 451, in draw
    self.figure.draw(self.renderer)
  File "/Users/bdesham/Code/virtualenv/lib/python2.7/site-packages/matplotlib/artist.py", line 55, in draw_wrapper
    draw(artist, renderer, *args, **kwargs)
  File "/Users/bdesham/Code/virtualenv/lib/python2.7/site-packages/matplotlib/figure.py", line 1034, in draw
    func(*args)
  File "/Users/bdesham/Code/virtualenv/lib/python2.7/site-packages/matplotlib/artist.py", line 55, in draw_wrapper
    draw(artist, renderer, *args, **kwargs)
  File "/Users/bdesham/Code/virtualenv/lib/python2.7/site-packages/matplotlib/axes.py", line 2086, in draw
    a.draw(renderer)
  File "/Users/bdesham/Code/virtualenv/lib/python2.7/site-packages/matplotlib/artist.py", line 55, in draw_wrapper
    draw(artist, renderer, *args, **kwargs)
  File "/Users/bdesham/Code/virtualenv/lib/python2.7/site-packages/matplotlib/text.py", line 2002, in draw
    xy_pixel = self._get_position_xy(renderer)
  File "/Users/bdesham/Code/virtualenv/lib/python2.7/site-packages/matplotlib/text.py", line 1640, in _get_position_xy
    return self._get_xy(renderer, x, y, self.xycoords)
  File "/Users/bdesham/Code/virtualenv/lib/python2.7/site-packages/matplotlib/text.py", line 1457, in _get_xy
    if s1 == 'data':
  File "/Users/bdesham/Code/virtualenv/lib/python2.7/site-packages/matplotlib/transforms.py", line 1562, in __eq__
    if other.is_affine:
AttributeError: 'str' object has no attribute 'is_affine'

What am I doing wrong?

Answer 1

I can't reproduce your error, however what you are trying to do can be done much more directly:

fig, ax = plt.subplots()

ax.plot([.5], [.5], 'bo')
ax.annotate(u'tt', xy=(.5, .5), xytext=(-6, -6), textcoords='offset points')

with out the need for you to do the scaling with dpi yourself.

In the future, can you post minimal runable examples with your question?