Python 实用宝典

Question 1

In Python, with Matplotlib, how can a scatter plot with empty circles be plotted? The goal is to draw empty circles around some of the colored disks already plotted by scatter(), so as to highlight them, ideally without having to redraw the colored circles.

I tried facecolors=None, to no avail.

Question 2

From the documentation for scatter:

Optional kwargs control the Collection properties; in particular:

    edgecolors:
        The string ‘none’ to plot faces with no outlines
    facecolors:
        The string ‘none’ to plot unfilled outlines

Try the following:

import matplotlib.pyplot as plt 
import numpy as np 

x = np.random.randn(60) 
y = np.random.randn(60)

plt.scatter(x, y, s=80, facecolors='none', edgecolors='r')
plt.show()

Note: For other types of plots see this post on the use of markeredgecolor and markerfacecolor.

Question 3

Would these work?

plt.scatter(np.random.randn(100), np.random.randn(100), facecolors='none')

or using plot()

plt.plot(np.random.randn(100), np.random.randn(100), 'o', mfc='none')

Question 4

Here’s another way: this adds a circle to the current axes, plot or image or whatever :

from matplotlib.patches import Circle  # $matplotlib/patches.py

def circle( xy, radius, color="lightsteelblue", facecolor="none", alpha=1, ax=None ):
    """ add a circle to ax= or current axes
    """
        # from .../pylab_examples/ellipse_demo.py
    e = Circle( xy=xy, radius=radius )
    if ax is None:
        ax = pl.gca()  # ax = subplot( 1,1,1 )
    ax.add_artist(e)
    e.set_clip_box(ax.bbox)
    e.set_edgecolor( color )
    e.set_facecolor( facecolor )  # "none" not None
    e.set_alpha( alpha )

(The circles in the picture get squashed to ellipses because imshow aspect="auto" ).

Question 5

In matplotlib 2.0 there is a parameter called fillstyle which allows better control on the way markers are filled. In my case I have used it with errorbars but it works for markers in general http://matplotlib.org/api/_as_gen/matplotlib.axes.Axes.errorbar.html

There are two important things to keep in mind when using fillstyle,

1) If mfc is set to any kind of value it will take priority, hence, if you did set fillstyle to ‘none’ it would not take effect. So avoid using mfc in conjuntion with fillstyle

2) You might want to control the marker edge width (using markeredgewidth or mew) because if the marker is relatively small and the edge width is thick, the markers will look like filled even though they are not.

Following is an example using errorbars:

myplot.errorbar(x=myXval, y=myYval, yerr=myYerrVal, fmt='o', fillstyle='none', ecolor='blue',  mec='blue')

Question 6

Basend on the example of Gary Kerr and as proposed here one may create empty circles related to specified values with following code:

import matplotlib.pyplot as plt 
import numpy as np 
from matplotlib.markers import MarkerStyle

x = np.random.randn(60) 
y = np.random.randn(60)
z = np.random.randn(60)

g=plt.scatter(x, y, s=80, c=z)
g.set_facecolor('none')
plt.colorbar()
plt.show()

Question 7

So I assume you want to highlight some points that fit a certain criteria. You can use Prelude’s command to do a second scatter plot of the hightlighted points with an empty circle and a first call to plot all the points. Make sure the s paramter is sufficiently small for the larger empty circles to enclose the smaller filled ones.

The other option is to not use scatter and draw the patches individually using the circle/ellipse command. These are in matplotlib.patches, here is some sample code on how to draw circles rectangles etc.

Question 8

I want to make a scatterplot (using matplotlib) where the points are shaded according to a third variable. I’ve got very close with this:

plt.scatter(w, M, c=p, marker='s')

where w and M are the datapoints and p is the variable I want to shade with respect to.
However I want to do it in greyscale rather than colour. Can anyone help?

Question 9

There’s no need to manually set the colors. Instead, specify a grayscale colormap…

import numpy as np
import matplotlib.pyplot as plt

# Generate data...
x = np.random.random(10)
y = np.random.random(10)

# Plot...
plt.scatter(x, y, c=y, s=500)
plt.gray()

plt.show()

Or, if you’d prefer a wider range of colormaps, you can also specify the cmap kwarg to scatter. To use the reversed version of any of these, just specify the “_r” version of any of them. E.g. gray_r instead of gray. There are several different grayscale colormaps pre-made (e.g. gray, gist_yarg, binary, etc).

import matplotlib.pyplot as plt
import numpy as np

# Generate data...
x = np.random.random(10)
y = np.random.random(10)

plt.scatter(x, y, c=y, s=500, cmap='gray')
plt.show()

Question 10

In matplotlib grey colors can be given as a string of a numerical value between 0-1.
For example c = '0.1'

Then you can convert your third variable in a value inside this range and to use it to color your points.
In the following example I used the y position of the point as the value that determines the color:

from matplotlib import pyplot as plt

x = [1, 2, 3, 4, 5, 6, 7, 8, 9]
y = [125, 32, 54, 253, 67, 87, 233, 56, 67]

color = [str(item/255.) for item in y]

plt.scatter(x, y, s=500, c=color)

plt.show()

Question 11

Sometimes you may need to plot color precisely based on the x-value case. For example, you may have a dataframe with 3 types of variables and some data points. And you want to do following,

Plot points corresponding to Physical variable ‘A’ in RED.
Plot points corresponding to Physical variable ‘B’ in BLUE.
Plot points corresponding to Physical variable ‘C’ in GREEN.

In this case, you may have to write to short function to map the x-values to corresponding color names as a list and then pass on that list to the plt.scatter command.

x=['A','B','B','C','A','B']
y=[15,30,25,18,22,13]

# Function to map the colors as a list from the input list of x variables
def pltcolor(lst):
    cols=[]
    for l in lst:
        if l=='A':
            cols.append('red')
        elif l=='B':
            cols.append('blue')
        else:
            cols.append('green')
    return cols
# Create the colors list using the function above
cols=pltcolor(x)

plt.scatter(x=x,y=y,s=500,c=cols) #Pass on the list created by the function here
plt.grid(True)
plt.show()

Python 实用宝典

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