From 6890b12a8fef925078e467e20cdac94bfdbea834 Mon Sep 17 00:00:00 2001
From: siriuspal <github@siriuspal.com>
Date: Fri, 12 Feb 2021 09:20:39 +0530
Subject: [PATCH] Added magnetic data cursor

---
 examples/draggable_magnetic_example.py | 19 ++++++++++++++++
 mpldatacursor/datacursor.py            | 30 +++++++++++++++++++++++---
 2 files changed, 46 insertions(+), 3 deletions(-)
 create mode 100644 examples/draggable_magnetic_example.py

diff --git a/examples/draggable_magnetic_example.py b/examples/draggable_magnetic_example.py
new file mode 100644
index 0000000..3be807f
--- /dev/null
+++ b/examples/draggable_magnetic_example.py
@@ -0,0 +1,19 @@
+"""
+This example demonstrates draggable annotation boxes, using the
+``display="multiple"`` option and ``magnetic="True"`` option.
+Magnetic option will cause pointer to stick to nearest datapoint
+instead of anywhere on the line.
+"""
+import matplotlib.pyplot as plt
+import numpy as np
+from mpldatacursor import datacursor
+
+data = np.outer(range(10), range(1, 5))
+
+fig, ax = plt.subplots()
+ax.set_title('Try clicking in between data points')
+ax.plot(data, 'o-')
+
+datacursor(display='multiple', draggable=True, magnetic=True)
+
+plt.show()
diff --git a/mpldatacursor/datacursor.py b/mpldatacursor/datacursor.py
index 88bbaa5..5eb1c91 100644
--- a/mpldatacursor/datacursor.py
+++ b/mpldatacursor/datacursor.py
@@ -58,7 +58,7 @@ class DataCursor(object):
     def __init__(self, artists, tolerance=5, formatter=None, point_labels=None,
                  display='one-per-axes', draggable=False, hover=False,
                  props_override=None, keybindings=True, date_format='%x %X',
-                 display_button=1, hide_button=3, keep_inside=True,
+                 display_button=1, hide_button=3, keep_inside=True, magnetic=False,
                  **kwargs):
         """Create the data cursor and connect it to the relevant figure.
 
@@ -125,6 +125,11 @@ def __init__(self, artists, tolerance=5, formatter=None, point_labels=None,
             the figure. This option has no effect on draggable datacursors.
             Defaults to True. Note: Currently disabled on OSX and
             NbAgg/notebook backends.
+        magnetic: boolean, optional
+            Magnetic will attach the cursor only to the data points.
+            Default is cursor can be added to interpolated lines.
+            If exact data point is not clicked, nearby data point will be selected.
+            Works with artists that have x, y attributes. Other plots will ignore Magnetic.
         **kwargs : additional keyword arguments, optional
             Additional keyword arguments are passed on to annotate.
         """
@@ -171,6 +176,7 @@ def filter_artists(artists):
             self.display = 'single'
             self.draggable = False
 
+        self.magnetic = magnetic
         self.keep_inside = keep_inside
         self.tolerance = tolerance
         self.point_labels = point_labels
@@ -704,6 +710,20 @@ def contains(artist, event):
             else:
                 return False, {}
 
+        def magnetic_datapoint_adjustment(event, artist):
+            """Updates the event coordinates to one of the closest data points"""
+
+            try:
+                # Get closest data point of x-axis
+                x = min(artist._x, key=lambda x: abs(x-event.xdata))
+                # Identify index of x value in _x and then get y value from _y
+                y = artist._y[list(artist._x).index(x)]
+            # If artist do not have x and y attributes, example Image, PathCollection
+            except AttributeError:
+                pass
+            else:
+                event.xdata, event.ydata = x, y
+
         # If we're on top of an annotation box, hide it if right-clicked or
         # do nothing if we're in draggable mode
         for anno in list(self.annotations.values()):
@@ -719,8 +739,12 @@ def contains(artist, event):
             inside, info = contains(artist, fixed_event)
             if inside:
                 fig = artist.figure
-                new_event = PickEvent('pick_event', fig.canvas, fixed_event,
-                                     artist, **info)
+                
+                # If magnetic is True, update event to closest data points
+                if self.magnetic:
+                    magnetic_datapoint_adjustment(fixed_event, artist)
+
+                new_event = PickEvent('pick_event', fig.canvas, fixed_event, artist, **info)
                 self(new_event)
 
                 # Only fire a single pick event for one mouseevent. Otherwise