Question

According to the documentation page:
http://matplotlib.sourceforge.net/api/pyplot_api.html
the way to use the axvline is like

axvline(x=0, ymin=0, ymax=1)

However, this does not work in my computer. Nothing is drawn. Rather, simply

axvline(x=0)

without setting the ymin and ymax works.

I am not sure whether this is a bug. Or maybe I missed something subtle?

matplotlib.__version__
'0.99.1.1'

uname -a
Linux pc20172 2.6.32-41-generic #94-Ubuntu SMP Fri Jul 6 18:00:34 UTC 2012 x86_64 GNU/Linux

Edit: a minimum code to reproduce the problem.

from pylab import *
ion()
plot([1,2])
axvline(x=0.5, ymin=1, ymax=2) # No vertical line is drawn.

clf() # Clear the figure to redo the plot.
plot([1,2])
axvline(x=0.5) # Now the desired vertical line is drawn.

Answer 1

From the documentation in help(axvline):

Draw a vertical line at x from ymin to ymax. With the default
values of ymin = 0 and ymax = 1, this line will always span the
vertical extent of the axes, regardless of the ylim settings, even if
you change them, eg. with the :meth:set_ylim command. That is,
the vertical extent is in axes coords: 0=bottom, 0.5=middle, 1.0=top
but the x location is in data coordinates.

So

axvline(x=0.5, ymin=1, ymax=2) # No vertical line is drawn.

is drawing a line just off the plot area. If you make the linewidth bigger, you can see this: