如何使Matplotlib散点图作为一个组透明? [英] How to make Matplotlib scatterplots transparent as a group?

问题描述

我正在使用 Matplotlib(python 3.4.0，matplotlib 1.4.3，在 Linux Mint 17 上运行)制作一些散点图.为每个点单独设置 alpha 透明度很容易；有什么办法可以将它们设置为一组，使同一组中的两个重叠点不改变颜色?

I'm making some scatterplots using Matplotlib (python 3.4.0, matplotlib 1.4.3, running on Linux Mint 17). It's easy enough to set alpha transparency for each point individually; is there any way to set them as a group, so that two overlapping points from the same group don't change the color?

示例代码:

import matplotlib.pyplot as plt
import numpy as np

def points(n=100):
    x = np.random.uniform(size=n)
    y = np.random.uniform(size=n)
    return x, y
x1, y1 = points()
x2, y2 = points()
fig = plt.figure(figsize=(4,4))
ax = fig.add_subplot(111, title="Test scatter")
ax.scatter(x1, y1, s=100, color="blue", alpha=0.5)
ax.scatter(x2, y2, s=100, color="red", alpha=0.5)
fig.savefig("test_scatter.png")

结果如下:

但我想要更像这样的:

我可以通过另存为 SVG 并在 Inkscape 中手动分组，然后设置透明度来解决，但我真的更喜欢我可以编码的东西.有什么建议吗?

I can workaround by saving as SVG and manually grouping then in Inkscape, then setting transparency, but I'd really prefer something I can code. Any suggestions?

推荐答案

是的，有趣的问题.您可以使用 Shapely 获得此散点图.这是代码:

Yes, interesting question. You can get this scatterplot with Shapely. Here is the code :

import matplotlib.pyplot as plt
import matplotlib.patches as ptc
import numpy as np
from shapely.geometry import Point
from shapely.ops import cascaded_union

n = 100
size = 0.02
alpha = 0.5

def points():
    x = np.random.uniform(size=n)
    y = np.random.uniform(size=n)
    return x, y

x1, y1 = points()
x2, y2 = points()
polygons1 = [Point(x1[i], y1[i]).buffer(size) for i in range(n)]
polygons2 = [Point(x2[i], y2[i]).buffer(size) for i in range(n)]
polygons1 = cascaded_union(polygons1)
polygons2 = cascaded_union(polygons2)

fig = plt.figure(figsize=(4,4))
ax = fig.add_subplot(111, title="Test scatter")
for polygon1 in polygons1:
    polygon1 = ptc.Polygon(np.array(polygon1.exterior), facecolor="red", lw=0, alpha=alpha)
    ax.add_patch(polygon1)
for polygon2 in polygons2:
    polygon2 = ptc.Polygon(np.array(polygon2.exterior), facecolor="blue", lw=0, alpha=alpha)
    ax.add_patch(polygon2)
ax.axis([-0.2, 1.2, -0.2, 1.2])

fig.savefig("test_scatter.png")

结果是:

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