I noticed when I pass the mouse pointer over a point on
> a graph that the (X, Y) values are displayed. This is
> *great*.
Glad you like it. Fernando Perez pushed me into getting this
implemented by repeatedly glowing about how nice this was in
gnuplot ... No better way to get me to implement a feature than
favorably compare gnuplot to matplotlib 
> What if I wanted to customize/extend what gets printed
> in response to mouse pointer position? Is this
> possible?
Yes, you can use the fmt_xdata and fmt_ydata attributes of the axes
instance to set a custom function. If these are not set, matplotlib
will fall back on the default tick formatter -- these are explained at
http://matplotlib.sf.net/matplotlib.ticker.html . See
examples/date_demo1.py for an example of setting your own coordinate
formatter functions with fmt_xdata and fmt_ydata. The basic usage is
def price(x): return '$%1.2f'%x
ax.fmt_xdata = DateFormatter('%Y-%m-%d')
ax.fmt_ydata = price
Here fmt_xdata is set to a callable instance (a DateFormatter
instance) and fmt_ydata is a plain old function that takes y as a
value and returns a string.
> e.g. If you had say 5 graphs on one plot, could you
> display all "Y values" for every X value??
> (X, Y1, Y2, Y3, Y4, Y5)??
This is a harder problem. You need to capture the mouse location (see
examples/coords_demo.py) to get the x location and use this
information in your y formatter function . Basically, you want to
ignore the actual value passed to your y format function and use the y
values of your time series at a given x index which you capture with a
mouse move event.
In the example below I use a class to store the x location and
communicate between the mouse position and the y format function. The
variable self.ind is the index into the x, y1 and y2 vectors that is
closest to the mouse x position. The example assumes your xdata are
monotonic, though you could do it for non-monotonic x with minor
modifications.
Note in principle the example below should work on all matplotlib
backends and os platforms. In practice, with a little testing, I
found that this example reveals some bugs in matplotlib event handling
which we are looking into. So for now, it appears to only work
properly on the GTK* backends (sigh), though it doesn't use any gtk
specific code.
from matplotlib.matlab import *
class MyFormatter:
def __init__(self, x, y1, y2):
self.x = x
self.y1 = y1
self.y2 = y2
self.ind = None
def on_move(self, event):
'save the last x coord and get the index'
if event.inaxes:
# get the index into x closest to
self.ind = searchsorted(self.x, event.xdata)
else:
self.ind = None
def fmty(self, y):
'ignore y and format the value of y1 and y2 at ind'
if self.ind is None: return ''
thisy1 = self.y1[self.ind]
thisy2 = self.y2[self.ind]
return '%1.2f, %1.2f'%(thisy1, thisy2)
t = arange(0, 2.0, 0.01)
s1 = sin(2*pi*t)
s2 = 2*sin(3*pi*t)
plot(t, s1, t, s2)
canvas = get_current_fig_manager().canvas
ax = gca()
o = MyFormatter(t, s1, s2)
canvas.mpl_connect('motion_notify_event', o.on_move)
ax.fmt_ydata = o.fmty
show()