You'll want to use event handling to figure out where the user clicked, and then you have a couple of options: Axes.vlines(), or pylab.axvline(). It seems like pylab.axvline() will always span the entire y-axis by default, but with Axes.vlines() you need to specify the ymin/ymax. Maybe someone else knows of an argument to pass to Axes.vlines() that will always span the entire y-axis.

Here's the code (assuming 'ipython -pylab'):
========

fig = figure()
plot([1,2,3,4], [5,6,7,8])

def onclick(event):
    """Draw a vertical line spanning the axes every time the user clicks inside them"""
    if event.inaxes: # make sure the click was within a set of axes
        pylab.axvline(event.xdata, axes=event.inaxes, color='r', linestyle=':') # red dotted line
        event.inaxes.figure.canvas.draw() # force a re-draw

cid = fig.canvas.mpl_connect('button_press_event', onclick) # add the click handler

... interact with it

fig.canvas.mpl_disconnect(cid) # get rid of the click-handler

========
Docs:
Axes.vlines(): http://matplotlib.sourceforge.net/api/axes_api.html#matplotlib.axes.Axes.vlines
pyplot.axvline(): http://matplotlib.sourceforge.net/api/pyplot_api.html#matplotlib.pyplot.axvline

Event handling: http://matplotlib.sourceforge.net/users/event_handling.html
Example: http://matplotlib.sourceforge.net/examples/event_handling/data_browser.html

   Justin

On Mon, May 2, 2011 at 10:08 AM, Soumyaroop Roy <roy.soumyaroop@gmail.com> wrote:
Any pointers on this?

On Sat, Apr 30, 2011 at 12:34 AM, Soumyaroop Roy <roy.soumyaroop@gmail.com> wrote:
Hi there:

I have an x-y plot and I want to draw a vertical marker (an x=c line) on the plot on a mouse click.

How should I approach it?

regards,
Soumyaroop


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