Performance

Whether exploring data in interactive mode or programmatically saving lots of plots, rendering performance can be a challenging bottleneck in your pipeline. Matplotlib provides multiple ways to greatly reduce rendering time at the cost of a slight change (to a settable tolerance) in your plot's appearance. The methods available to reduce rendering time depend on the type of plot that is being created.

Line segment simplification

For plots that have line segments (e.g. typical line plots, outlines of polygons, etc.), rendering performance can be controlled by rcParams["path.simplify"] (default: True) and rcParams["path.simplify_threshold"] (default: 0.111111111111), which can be defined e.g. in the matplotlibrc file (see Customizing Matplotlib with style sheets and rcParams for more information about the matplotlibrc file). rcParams["path.simplify"] (default: True) is a Boolean indicating whether or not line segments are simplified at all. rcParams["path.simplify_threshold"] (default: 0.111111111111) controls how much line segments are simplified; higher thresholds result in quicker rendering.

The following script will first display the data without any simplification, and then display the same data with simplification. Try interacting with both of them:

import numpy as np
import matplotlib.pyplot as plt
import matplotlib as mpl

# Setup, and create the data to plot
y = np.random.rand(100000)
y[50000:] *= 2
y[np.geomspace(10, 50000, 400).astype(int)] = -1
mpl.rcParams['path.simplify'] = True

mpl.rcParams['path.simplify_threshold'] = 0.0
plt.plot(y)
plt.show()

mpl.rcParams['path.simplify_threshold'] = 1.0
plt.plot(y)
plt.show()

Matplotlib currently defaults to a conservative simplification threshold of 1/9. To change default settings to use a different value, change the matplotlibrc file. Alternatively, users can create a new style for interactive plotting (with maximal simplification) and another style for publication quality plotting (with minimal simplification) and activate them as necessary. See Customizing Matplotlib with style sheets and rcParams for instructions on how to perform these actions.

The simplification works by iteratively merging line segments into a single vector until the next line segment's perpendicular distance to the vector (measured in display-coordinate space) is greater than the path.simplify_threshold parameter.

Note

Changes related to how line segments are simplified were made in version 2.1. Rendering time will still be improved by these parameters prior to 2.1, but rendering time for some kinds of data will be vastly improved in versions 2.1 and greater.

Marker simplification

Markers can also be simplified, albeit less robustly than line segments. Marker simplification is only available to Line2D objects (through the markevery property). Wherever Line2D construction parameters are passed through, such as matplotlib.pyplot.plot() and matplotlib.axes.Axes.plot(), the markevery parameter can be used:

plt.plot(x, y, markevery=10)

The markevery argument allows for naive subsampling, or an attempt at evenly spaced (along the x axis) sampling. See the Markevery Demo for more information.

Splitting lines into smaller chunks

If you are using the Agg backend (see What is a backend?), then you can make use of rcParams["agg.path.chunksize"] (default: 0) This allows users to specify a chunk size, and any lines with greater than that many vertices will be split into multiple lines, each of which has no more than agg.path.chunksize many vertices. (Unless agg.path.chunksize is zero, in which case there is no chunking.) For some kind of data, chunking the line up into reasonable sizes can greatly decrease rendering time.

The following script will first display the data without any chunk size restriction, and then display the same data with a chunk size of 10,000. The difference can best be seen when the figures are large, try maximizing the GUI and then interacting with them:

import numpy as np
import matplotlib.pyplot as plt
import matplotlib as mpl
mpl.rcParams['path.simplify_threshold'] = 1.0

# Setup, and create the data to plot
y = np.random.rand(100000)
y[50000:] *= 2
y[np.geomspace(10, 50000, 400).astype(int)] = -1
mpl.rcParams['path.simplify'] = True

mpl.rcParams['agg.path.chunksize'] = 0
plt.plot(y)
plt.show()

mpl.rcParams['agg.path.chunksize'] = 10000
plt.plot(y)
plt.show()

Legends

The default legend behavior for axes attempts to find the location that covers the fewest data points (loc='best'). This can be a very expensive computation if there are lots of data points. In this case, you may want to provide a specific location.

Using the fast style

The fast style can be used to automatically set simplification and chunking parameters to reasonable settings to speed up plotting large amounts of data. The following code runs it:

import matplotlib.style as mplstyle
mplstyle.use('fast')

It is very lightweight, so it works well with other styles. Be sure the fast style is applied last so that other styles do not overwrite the settings:

mplstyle.use(['dark_background', 'ggplot', 'fast'])