.. only:: html .. note:: :class: sphx-glr-download-link-note Click :ref:`here ` to download the full example code .. rst-class:: sphx-glr-example-title .. _sphx_glr_gallery_misc_cursor_demo.py: ================= 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 .. code-block:: default import matplotlib.pyplot as plt import numpy as np 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('x=%1.2f, y=%1.2f' % (x, y)) 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) .. image:: /gallery/misc/images/sphx_glr_cursor_demo_001.png :alt: Simple cursor :class: sphx-glr-single-img .. rst-class:: sphx-glr-script-out Out: .. code-block:: none 5 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'``. .. code-block:: default 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('x=%1.2f, y=%1.2f' % (x, y)) 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) .. image:: /gallery/misc/images/sphx_glr_cursor_demo_002.png :alt: Blitted cursor :class: sphx-glr-single-img .. rst-class:: sphx-glr-script-out Out: .. code-block:: none 6 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. .. code-block:: default 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('x=%1.2f, y=%1.2f' % (x, y)) 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) plt.show() .. image:: /gallery/misc/images/sphx_glr_cursor_demo_003.png :alt: Snapping cursor :class: sphx-glr-single-img .. _sphx_glr_download_gallery_misc_cursor_demo.py: .. only :: html .. container:: sphx-glr-footer :class: sphx-glr-footer-example .. container:: sphx-glr-download sphx-glr-download-python :download:`Download Python source code: cursor_demo.py ` .. container:: sphx-glr-download sphx-glr-download-jupyter :download:`Download Jupyter notebook: cursor_demo.ipynb ` .. only:: html .. rst-class:: sphx-glr-signature Keywords: matplotlib code example, codex, python plot, pyplot `Gallery generated by Sphinx-Gallery `_