CODEWITHSUNDEEP
pythonmatplotlibplotlegend
BlueMountain
BlueMountain

Reputation: 247

Matplotlib legend makes the image too large

I'm plotting this figure with matplotlib, the for loop just color the background:

fig, ax = plt.subplots()

ax.set_ylabel('Number of contacts')  
ax.set_xlabel('Time [s]') 

for m in range(len(data[node])):
    if data[node][m] == -1:
        ax.axvline(m,color='r',linewidth=5,alpha=0.2,label="OUT")
    if data[node][m] == 0:
        ax.axvline(m,color='g',linewidth=5,alpha=0.2,label="RZ0")
    if data[node][m] == 1:
        ax.axvline(m,color='y',linewidth=5,alpha=0.2,label="RZ1")

ax.plot(x, y, 'b+')
# ax.legend() # HERE is the problem
plt.show()

Which plots the following:

enter image description here

What I want now is a legend to indicate each color of the background meaning, but when I include ax.legend() I get the following error:

ValueError: Image size of 392x648007 pixels is too large. It must be less than 2^16 in each 
direction.

<Figure size 432x288 with 1 Axes>
<Figure size 432x288 with 0 Axes>

How am I supposed to name each color of the background, there are 43200 vertical lines but only 3 colors, does it have anything to do with the number of lines?

Upvotes: 0

Views: 1454

Answers (2)

JohanC
JohanC

Reputation: 80329

The trick is to set the label only once. You can add a variable for each label and replace it with None once it's used. Note that using axvline to draw a background has the problem that the line width is measured in pixel space, so neighboring lines will either overlap or have a small white space inbetween. Better to use axvspan. To avoid the white space at the left and at the right, you can explicitly set the x-limits.

The code can be simplified somewhat using a loop.

Updated code:

  • group consecutive spans together for drawing
  • precalculate the effect of alpha so the background can be drawn without the need for transparency
from matplotlib import pyplot as plt
from matplotlib import colors as mcolors
import numpy as np
import pandas as pd
import itertools

fig, ax = plt.subplots()

# create some random data
x = np.arange(100)
y = np.sinh(x/20)
indicators = [-1, 0, 1]
node = 0
data = [np.random.choice(indicators, len(x), p=[10/16,1/16,5/16])]

labels = ["OUT", "RZ0", "RZ1"]
colors = ['lime', 'purple', 'gold']

alpha = 0.4
# precalculate the effect of alpha so the colors can be applied with alpha=1
colors = [[1 + (x - 1) * alpha for x in mcolors.to_rgb(c)] for c in colors]

m = 0
for val, group in itertools.groupby(data[node]):
    width = len(list(group))
    ind = indicators.index(val)
    ax.axvspan(m, m + width, color=colors[ind], linewidth=0, alpha=1, label=labels[ind])
    labels[ind] = None  # reset the label to make sure it is only used once
    m += width

ax.plot(x, y, 'b+')
ax.set_xlim(0, len(data[node]))
ax.legend(framealpha=1) # to make the legend background opaque
plt.show()

sample plot

Upvotes: 1

Jody Klymak
Jody Klymak

Reputation: 5913

Do something like hrz1 = ax.axvline(m,color='y',linewidth=5,alpha=0.2) for each of your classes, and then ax.legend((hrz1, hrz0, hout), ('RZ1', 'RZ0', 'OUT'). The hrz1 pointer will be rewritten for each line you make, and then legend will only make one label for each of the handles.

Upvotes: 1

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