Matplotlib cursor value with two axes

Question

I used twinx() to get two y-axes on the plot. However, I'd like the navigation bar to report the y-coordinates of the first axis. It seems by default, it reports the position to the second axis. How do you change it to report the first axis, or better yet, report both?

Answer 1

The navigation bar shows the coordinates of the Axes that is on top. Hence, all you have to do is to make the zorder of the right Axes lower than that of the left Axes:

ax2.set_zorder(-100)

This will also change the appearance of the Figure. That is, everything plotted on the right Axes will go behind everything plotted on the left Axes. In many cases this makes very little or no difference.

Answer 2

Given help from tcaswell's answers (here, and here), you could modify the tracker to display coordinates with respect to both axes like this:

import numpy as np
import matplotlib.pyplot as plt
import matplotlib.cm as cm

def make_format(current, other):
    # current and other are axes
    def format_coord(x, y):
        # x, y are data coordinates
        # convert to display coords
        display_coord = current.transData.transform((x,y))
        inv = other.transData.inverted()
        # convert back to data coords with respect to ax
        ax_coord = inv.transform(display_coord)
        coords = [ax_coord, (x, y)]
        return ('Left: {:<40}    Right: {:<}'
                .format(*['({:.3f}, {:.3f})'.format(x, y) for x,y in coords]))
    return format_coord

np.random.seed(6)
numdata = 100
t = np.linspace(0.05, 0.11, numdata)
y1 = np.cumsum(np.random.random(numdata) - 0.5) * 40000
y2 = np.cumsum(np.random.random(numdata) - 0.5) * 0.002

fig = plt.figure()

ax1 = fig.add_subplot(111)
ax2 = ax1.twinx()

ax2.format_coord = make_format(ax2, ax1)

ax1.plot(t, y1, 'r-', label='y1')
ax2.plot(t, y2, 'g-', label='y2')

plt.show()

Alternatively, if you have scipy, you could use FollowDotCursor, which will annotate the data point closest to the cursor. Done this way, the user's eyes do not have to move far from the graph to find the data coordinates. Moreover, it can be applied to more than two artists (just add a FollowDotCursor for each line, scatter plot, bar graph, etc.). It is also more accurate since the annotation window shows the values of the closest data point, not simply the coordinates of the cursor in data coordinates.

import numpy as np
import matplotlib.pyplot as plt
import matplotlib.cm as cm
import scipy.spatial as spatial

def fmt(x, y):
    return 'x: {x:0.2f}\ny: {y:0.2f}'.format(x=x, y=y)

class FollowDotCursor(object):
    """Display the x,y location of the nearest data point.
    https://stackoverflow.com/a/4674445/190597 (Joe Kington)
    https://stackoverflow.com/a/20637433/190597 (unutbu)
    """
    def __init__(self, ax, x, y, formatter=fmt, offsets=(-20, 20)):
        try:
            x = np.asarray(x, dtype='float')
        except (TypeError, ValueError):
            x = np.asarray(mdates.date2num(x), dtype='float')
        y = np.asarray(y, dtype='float')
        mask = ~(np.isnan(x) | np.isnan(y))
        x = x[mask]
        y = y[mask]
        self._points = np.column_stack((x, y))
        self.offsets = offsets
        y = y[np.abs(y - y.mean()) <= 3 * y.std()]
        self.scale = x.ptp()
        self.scale = y.ptp() / self.scale if self.scale else 1
        self.tree = spatial.cKDTree(self.scaled(self._points))
        self.formatter = formatter
        self.ax = ax
        self.fig = ax.figure
        self.ax.xaxis.set_label_position('top')
        self.dot = ax.scatter(
            [x.min()], [y.min()], s=130, color='green', alpha=0.7)
        self.annotation = self.setup_annotation()
        plt.connect('motion_notify_event', self)

    def scaled(self, points):
        points = np.asarray(points)
        return points * (self.scale, 1)

    def __call__(self, event):
        ax = self.ax
        # event.inaxes is always the current axis. If you use twinx, ax could be
        # a different axis.
        if event.inaxes == ax:
            x, y = event.xdata, event.ydata
        elif event.inaxes is None:
            return
        else:
            inv = ax.transData.inverted()
            x, y = inv.transform([(event.x, event.y)]).ravel()
        annotation = self.annotation
        x, y = self.snap(x, y)
        annotation.xy = x, y
        annotation.set_text(self.formatter(x, y))
        self.dot.set_offsets((x, y))
        event.canvas.draw()

    def setup_annotation(self):
        """Draw and hide the annotation box."""
        annotation = self.ax.annotate(
            '', xy=(0, 0), ha = 'right',
            xytext = self.offsets, textcoords = 'offset points', va = 'bottom',
            bbox = dict(
                boxstyle='round,pad=0.5', fc='yellow', alpha=0.75),
            arrowprops = dict(
                arrowstyle='->', connectionstyle='arc3,rad=0'))
        return annotation

    def snap(self, x, y):
        """Return the value in self.tree closest to x, y."""
        dist, idx = self.tree.query(self.scaled((x, y)), k=1, p=1)
        try:
            return self._points[idx]
        except IndexError:
            # IndexError: index out of bounds
            return self._points[0]


np.random.seed(6)
numdata = 100
t = np.linspace(0.05, 0.11, numdata)
y1 = np.cumsum(np.random.random(numdata) - 0.5) * 40000
y2 = np.cumsum(np.random.random(numdata) - 0.5) * 0.002

fig = plt.figure()
ax1 = fig.add_subplot(111)
ax2 = ax1.twinx()

ax1.plot(t, y1, 'r-', label='y1')
ax2.plot(t, y2, 'g-', label='y2')

cursor1 = FollowDotCursor(ax1, t, y1)
cursor2 = FollowDotCursor(ax2, t, y2)
plt.show()