Legend placement bug

[dmcdougall]

This script produces a legend that covers the red point:

import matplotlib.pyplot as plt

colors = ['b','g','r']
for n in range(3):
    plt.scatter([n,],[n,],color=colors[n])
plt.legend(['foo','foo','foo'],loc='best')
plt.gca().set_xlim(-0.5, 2.2)
plt.gca().set_ylim(-0.5, 2.2)
plt.show()

[pelson]

Fantastic simple, reproducible case.

The problem is that the best placement only takes axes.lines into account. To get to the actual code, look at legend.py, and in particular the _find_best_position and _auto_legend_data methods.

[dmcdougall]

@pelson Thanks for that info. Given your comment, would it be too presumptuous to say we'd also see this behaviour for text and arrows under the legend? Perhaps _find_best_position and _auto_legend_data should look more broadly at the axes artists?

Edit: Grammar.

[pelson]

Perhaps _find_best_position and _auto_legend_data should look more broadly at the axes artists

In my eyes, that would make best better...

[dmcdougall]

So I'm trying to fix this just for the scatter case, so I can see what's going on. I'm having problems getting the bounding box for each of the points. This is my loop:

for handle in ax.collections:
    paths = handle.get_paths()
    transforms = handle.get_transforms()
    for p, t in zip(paths, transforms):
        box = p.get_extents(transform=t)
        print(box)

There are 3 PathCollections, presumably one for each point in the scatter plot, but I'm getting rubbish bounding boxes:

Bbox('array([[-2.23606798, -2.23606798],\n       [ 2.23606798,  2.23606798]])')
Bbox('array([[-2.23606798, -2.23606798],\n       [ 2.23606798,  2.23606798]])')
Bbox('array([[-2.23606798, -2.23606798],\n       [ 2.23606798,  2.23606798]])')
Bbox('array([[-2.23606798, -2.23606798],\n       [ 2.23606798,  2.23606798]])')
Bbox('array([[-2.23606798, -2.23606798],\n       [ 2.23606798,  2.23606798]])')
Bbox('array([[-2.23606798, -2.23606798],\n       [ 2.23606798,  2.23606798]])')

Any idea what's going on? Did I do something silly?

[pelson]

There are 3 PathCollections, presumably one for each point in the scatter plot

The way this code has been written, yes, there should be 3 collections. In most cases though, one would plot the 3 points in one PathCollection i.e. plt.scatter(range(3), range(3)). It may be that the bounding box for a collection cannot be broken down for each of its components.

My knowledge of Collections is very limited, but you may be interested in the _offsets and _transOffset attributes on a Collection...

[dmcdougall]

Aha! Thank you. So a PathCollection is actually just a single path, together with a bunch of offsets. Then that's enough information to define the whole collection. So it should be easy to get the bounding box for each component in the collection: Work out the bounding box of the path. Then, for each offset, transform that bounding box by the offset amount.

[pelson]

So it should be easy to get the bounding box for each component in the collection: Work out the bounding box of the path. Then, for each offset, transform that bounding box by the offset amount.

Easy when you put it like that :-)

[wmak]

Hey, I took a crack at solving this bug along with some help from @abalmeida7 and @ShaiMitchell
I used the _offsets from the collections that @dmcdougall presented above, and just added one to the badness score if the point intersected with the legend box.

@@ -716,6 +716,7 @@ class Legend(Artist):
         ax = self.parent
         bboxes = []
         lines = []
+        points = []

         for handle in ax.lines:
             assert isinstance(handle, Line2D)
@@ -734,12 +735,15 @@ class Legend(Artist):
                 transform = handle.get_transform()
                 bboxes.append(handle.get_path().get_extents(transform))

+        for handle in ax.collections:
+            points.append(handle._offsets)
+
         try:
             vertices = np.concatenate([l.vertices for l in lines])
         except ValueError:
             vertices = np.array([])

-        return [vertices, bboxes, lines]
+        return [vertices, bboxes, lines, points]

     def draw_frame(self, b):
         'b is a boolean.  Set draw frame to b'
@@ -896,7 +900,7 @@ class Legend(Artist):
         # should always hold because function is only called internally
         assert self.isaxes

-        verts, bboxes, lines = self._auto_legend_data()
+        verts, bboxes, lines, points = self._auto_legend_data()

         bbox = Bbox.from_bounds(0, 0, width, height)
         if consider is None:
@@ -927,12 +927 @@ class Legend(Artist):
                 if line.intersects_bbox(legendBox):
                     badness += 1

+            for point in points:
+                val = self.parent.transData.transform(np.vstack(point[0]).T)
+                if legendBox.count_contains(val):
+                    badness += 1
             ox, oy = l, b
             if badness == 0:
                 return ox, oy

             candidates.append((badness, (l, b)))

[tacaswell]

@wmak Could you put that in a PR? It make reviewing proposed changes much easier (and lets the CI server have at it).