cursor tracking using matplotlib and twinx

Question

I would like to track the coordinates of the mouse with respect to data coordinates on two axes simultaneously. I can track the mouse position with respect to one a

Answer 1

You can track both axis coordinates with one cursor (or event handler) this way:

import numpy as np
import matplotlib.pyplot as plt
import logging
logger = logging.getLogger(__name__)

class Cursor(object):
    def __init__(self):
        plt.connect('motion_notify_event', self)

    def __call__(self, event):
        if event.inaxes is None:
            return
        x, y1 = ax1.transData.inverted().transform((event.x,event.y))
        x, y2 = ax2.transData.inverted().transform((event.x,event.y))
        logger.debug('(x,y1,y2)=({x:0.2f}, {y1:0.2f}, {y2:0.2f})'.format(x=x,y1=y1,y2=y2))

logging.basicConfig(level=logging.DEBUG,
                    format='%(message)s',)
fig, ax1 = plt.subplots()

x = np.linspace(1000, 2000, 500)
y = 100*np.sin(20*np.pi*(x-1500)/2000.0)
fern = Cursor()
ax1.plot(x,y)
ax2 = ax1.twinx()
z = x/200.0
ax2.plot(x,z)
plt.show()

(I got "too many indices" when I used ax2.semilogy(x,z) like the OP, but didn't work through that problem.)

The ax1.transData.inverted().transform((event.x,event.y)) code performs a transform from display to data coordinates on the specified axis and can be used with either axis at will.