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"""
=================
Cross-hair cursor
=================
This example adds a cross-hair as a data cursor. The cross-hair is
implemented as regular line objects that are updated on mouse move.
We show three implementations:
1) A simple cursor implementation that redraws the figure on every mouse move.
This is a bit slow, and you may notice some lag of the cross-hair movement.
2) A cursor that uses blitting for speedup of the rendering.
3) A cursor that snaps to data points.
Faster cursoring is possible using native GUI drawing, as in
:doc:`/gallery/user_interfaces/wxcursor_demo_sgskip`.
The mpldatacursor__ and mplcursors__ third-party packages can be used to
achieve a similar effect.
__ https://github.com/joferkington/mpldatacursor
__ https://github.com/anntzer/mplcursors
.. redirect-from:: /gallery/misc/cursor_demo
"""
import matplotlib.pyplot as plt
import numpy as np
from matplotlib.backend_bases import MouseEvent
class Cursor:
"""
A cross hair cursor.
"""
def __init__(self, ax):
self.ax = ax
self.horizontal_line = ax.axhline(color='k', lw=0.8, ls='--')
self.vertical_line = ax.axvline(color='k', lw=0.8, ls='--')
# text location in axes coordinates
self.text = ax.text(0.72, 0.9, '', transform=ax.transAxes)
def set_cross_hair_visible(self, visible):
need_redraw = self.horizontal_line.get_visible() != visible
self.horizontal_line.set_visible(visible)
self.vertical_line.set_visible(visible)
self.text.set_visible(visible)
return need_redraw
def on_mouse_move(self, event):
if not event.inaxes:
need_redraw = self.set_cross_hair_visible(False)
if need_redraw:
self.ax.figure.canvas.draw()
else:
self.set_cross_hair_visible(True)
x, y = event.xdata, event.ydata
# update the line positions
self.horizontal_line.set_ydata([y])
self.vertical_line.set_xdata([x])
self.text.set_text(f'x={x:1.2f}, y={y:1.2f}')
self.ax.figure.canvas.draw()
x = np.arange(0, 1, 0.01)
y = np.sin(2 * 2 * np.pi * x)
fig, ax = plt.subplots()
ax.set_title('Simple cursor')
ax.plot(x, y, 'o')
cursor = Cursor(ax)
fig.canvas.mpl_connect('motion_notify_event', cursor.on_mouse_move)
# Simulate a mouse move to (0.5, 0.5), needed for online docs
t = ax.transData
MouseEvent(
"motion_notify_event", ax.figure.canvas, *t.transform((0.5, 0.5))
)._process()
# %%
# Faster redrawing using blitting
# """""""""""""""""""""""""""""""
# This technique stores the rendered plot as a background image. Only the
# changed artists (cross-hair lines and text) are rendered anew. They are
# combined with the background using blitting.
#
# This technique is significantly faster. It requires a bit more setup because
# the background has to be stored without the cross-hair lines (see
# ``create_new_background()``). Additionally, a new background has to be
# created whenever the figure changes. This is achieved by connecting to the
# ``'draw_event'``.
class BlittedCursor:
"""
A cross-hair cursor using blitting for faster redraw.
"""
def __init__(self, ax):
self.ax = ax
self.background = None
self.horizontal_line = ax.axhline(color='k', lw=0.8, ls='--')
self.vertical_line = ax.axvline(color='k', lw=0.8, ls='--')
# text location in axes coordinates
self.text = ax.text(0.72, 0.9, '', transform=ax.transAxes)
self._creating_background = False
ax.figure.canvas.mpl_connect('draw_event', self.on_draw)
def on_draw(self, event):
self.create_new_background()
def set_cross_hair_visible(self, visible):
need_redraw = self.horizontal_line.get_visible() != visible
self.horizontal_line.set_visible(visible)
self.vertical_line.set_visible(visible)
self.text.set_visible(visible)
return need_redraw
def create_new_background(self):
if self._creating_background:
# discard calls triggered from within this function
return
self._creating_background = True
self.set_cross_hair_visible(False)
self.ax.figure.canvas.draw()
self.background = self.ax.figure.canvas.copy_from_bbox(self.ax.bbox)
self.set_cross_hair_visible(True)
self._creating_background = False
def on_mouse_move(self, event):
if self.background is None:
self.create_new_background()
if not event.inaxes:
need_redraw = self.set_cross_hair_visible(False)
if need_redraw:
self.ax.figure.canvas.restore_region(self.background)
self.ax.figure.canvas.blit(self.ax.bbox)
else:
self.set_cross_hair_visible(True)
# update the line positions
x, y = event.xdata, event.ydata
self.horizontal_line.set_ydata([y])
self.vertical_line.set_xdata([x])
self.text.set_text(f'x={x:1.2f}, y={y:1.2f}')
self.ax.figure.canvas.restore_region(self.background)
self.ax.draw_artist(self.horizontal_line)
self.ax.draw_artist(self.vertical_line)
self.ax.draw_artist(self.text)
self.ax.figure.canvas.blit(self.ax.bbox)
x = np.arange(0, 1, 0.01)
y = np.sin(2 * 2 * np.pi * x)
fig, ax = plt.subplots()
ax.set_title('Blitted cursor')
ax.plot(x, y, 'o')
blitted_cursor = BlittedCursor(ax)
fig.canvas.mpl_connect('motion_notify_event', blitted_cursor.on_mouse_move)
# Simulate a mouse move to (0.5, 0.5), needed for online docs
t = ax.transData
MouseEvent(
"motion_notify_event", ax.figure.canvas, *t.transform((0.5, 0.5))
)._process()
# %%
# Snapping to data points
# """""""""""""""""""""""
# The following cursor snaps its position to the data points of a `.Line2D`
# object.
#
# To save unnecessary redraws, the index of the last indicated data point is
# saved in ``self._last_index``. A redraw is only triggered when the mouse
# moves far enough so that another data point must be selected. This reduces
# the lag due to many redraws. Of course, blitting could still be added on top
# for additional speedup.
class SnappingCursor:
"""
A cross-hair cursor that snaps to the data point of a line, which is
closest to the *x* position of the cursor.
For simplicity, this assumes that *x* values of the data are sorted.
"""
def __init__(self, ax, line):
self.ax = ax
self.horizontal_line = ax.axhline(color='k', lw=0.8, ls='--')
self.vertical_line = ax.axvline(color='k', lw=0.8, ls='--')
self.x, self.y = line.get_data()
self._last_index = None
# text location in axes coords
self.text = ax.text(0.72, 0.9, '', transform=ax.transAxes)
def set_cross_hair_visible(self, visible):
need_redraw = self.horizontal_line.get_visible() != visible
self.horizontal_line.set_visible(visible)
self.vertical_line.set_visible(visible)
self.text.set_visible(visible)
return need_redraw
def on_mouse_move(self, event):
if not event.inaxes:
self._last_index = None
need_redraw = self.set_cross_hair_visible(False)
if need_redraw:
self.ax.figure.canvas.draw()
else:
self.set_cross_hair_visible(True)
x, y = event.xdata, event.ydata
index = min(np.searchsorted(self.x, x), len(self.x) - 1)
if index == self._last_index:
return # still on the same data point. Nothing to do.
self._last_index = index
x = self.x[index]
y = self.y[index]
# update the line positions
self.horizontal_line.set_ydata([y])
self.vertical_line.set_xdata([x])
self.text.set_text(f'x={x:1.2f}, y={y:1.2f}')
self.ax.figure.canvas.draw()
x = np.arange(0, 1, 0.01)
y = np.sin(2 * 2 * np.pi * x)
fig, ax = plt.subplots()
ax.set_title('Snapping cursor')
line, = ax.plot(x, y, 'o')
snap_cursor = SnappingCursor(ax, line)
fig.canvas.mpl_connect('motion_notify_event', snap_cursor.on_mouse_move)
# Simulate a mouse move to (0.5, 0.5), needed for online docs
t = ax.transData
MouseEvent(
"motion_notify_event", ax.figure.canvas, *t.transform((0.5, 0.5))
)._process()
plt.show()
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