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Allow setting frequency units for psd plots #32211

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4b43e6a
Allow setting frequency units for psd
cdholmes Aug 14, 2026
52fe0fa
Update psd documentation, example, type hinting, release notes
cdholmes Aug 14, 2026
740c249
Rearrange psd arguments to minimize API impact
cdholmes Aug 16, 2026
b7a56e2
Remove assumed frequency units (Hz) from comments
cdholmes Aug 17, 2026
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6 changes: 6 additions & 0 deletions doc/api/next_api_changes/development/32211_CDH.rst
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@rcomer rcomer Aug 17, 2026

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next_api_changes is for changes that are not backwards compatible, so this is not needed. next_whats_new is the correct place to advertise this.

Original file line number Diff line number Diff line change
@@ -0,0 +1,6 @@
Power Spectral Density
~~~~~~~~~~~~~~~~~~~~~~
An optional keyword *Funits* is added to `.Axes.psd` so
units that differ from the default 'Hz' can be specified.
This change is backward compatible (no change if the optional
keyword is omitted.)
30 changes: 30 additions & 0 deletions doc/release/next_whats_new/new_psd_feature.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,30 @@
Sampling frequency units can be specified for `.Axes.psd`
---------------------------------------------------------------

@rcomer rcomer Aug 17, 2026

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Suggested change
---------------------------------------------------------------
---------------------------------------------------------

Should match the length of the title.


When creating a power spectral density (psd) plot, the units of the
sampling frequency can be specified. (Units were previously always
assumed to be Hz.)

::

@rcomer rcomer Aug 17, 2026

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If you use the plot directive here, you automatically get the image that this code produces. See here for an example

matplotlib/doc/release/next_whats_new/pie_wedge_labels.rst

Line 17 in 855008e

.. plot::

https://matplotlib.org/devdocs/release/next_whats_new/pie_wedge_labels.html


import matplotlib.pyplot as plt
import numpy as np

# Sampling period in units of days
dt = 1/24

# Create example signal: sinusoid with red noise
np.random.seed(19680801) # Fixing random state for reproducibility
t = np.arange(0, 20, dt)
nse = np.random.randn(len(t))
r = np.exp(-t / 0.05)
cnse = np.convolve(nse, r) * dt
cnse = cnse[:len(t)]
s = 0.1 * np.sin(2 * np.pi * t) + cnse

# Show signal and power spectral density
fig, (ax0, ax1) = plt.subplots(2, 1, layout='constrained')
ax0.plot(t,s)
ax0.set(xlabel='Time (d)', ylabel='Signal')
ax1.psd(s, NFFT=256, Fs=1 / dt, Funits='cpd')
plt.show()
3 changes: 3 additions & 0 deletions galleries/examples/statistics/psd_demo.py
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Original file line number Diff line number Diff line change
Expand Up @@ -33,6 +33,9 @@
ax0.set_xlabel('Time (s)')
ax0.set_ylabel('Signal')
ax1.psd(s, NFFT=512, Fs=1 / dt)
# If dt had other units (e.g. days instead of seconds),
# then the units of Fs (e.g. cycles per day or cps) can be specified
# by the keyword Funits (e.g. Funits='cpd') in psd.

plt.show()

Expand Down
13 changes: 10 additions & 3 deletions lib/matplotlib/axes/_axes.py
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Original file line number Diff line number Diff line change
Expand Up @@ -8113,7 +8113,8 @@ def ecdf(self, x, weights=None, *, complementary=False,
@_docstring.interpd
def psd(self, x, NFFT=None, Fs=None, Fc=None, detrend=None,
window=None, noverlap=None, pad_to=None,
sides=None, scale_by_freq=None, return_line=None, **kwargs):
sides=None, scale_by_freq=None, return_line=None, Funits=None,
**kwargs):
r"""
Plot the power spectral density.

Expand Down Expand Up @@ -8147,6 +8148,10 @@ def psd(self, x, NFFT=None, Fs=None, Fc=None, detrend=None,
return_line : bool, default: False
Whether to include the line object plotted in the returned values.

Funits : str, default: 'Hz'
Units for the sampling frequency *Fs*. It is used to label the
xaxis and yaxis.

@rcomer rcomer Aug 17, 2026

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This should have a versionadded directive.
https://matplotlib.org/devdocs/devel/api_changes.html#announce-new-and-deprecated-api

Returns
-------
Pxx : 1-D array
Expand Down Expand Up @@ -8194,19 +8199,21 @@ def psd(self, x, NFFT=None, Fs=None, Fc=None, detrend=None,
"""
if Fc is None:
Fc = 0
if Funits is None:
Funits = 'Hz'

pxx, freqs = mlab.psd(x=x, NFFT=NFFT, Fs=Fs, detrend=detrend,
window=window, noverlap=noverlap, pad_to=pad_to,
sides=sides, scale_by_freq=scale_by_freq)
freqs += Fc

if scale_by_freq in (None, True):
psd_units = 'dB/Hz'
psd_units = 'dB/%s' % Funits
else:
psd_units = 'dB'

line = self.plot(freqs, 10 * np.log10(pxx), **kwargs)
self.set_xlabel('Frequency')
self.set_xlabel('Frequency (%s)' % Funits)
self.set_ylabel('Power Spectral Density (%s)' % psd_units)
self.grid(True)

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1 change: 1 addition & 0 deletions lib/matplotlib/axes/_axes.pyi
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Original file line number Diff line number Diff line change
Expand Up @@ -654,6 +654,7 @@ class Axes(_AxesBase):
scale_by_freq: bool | None = ...,
return_line: bool | None = ...,
data: DataParamType = ...,
Funits: str | None = ...,
**kwargs
) -> tuple[np.ndarray, np.ndarray] | tuple[np.ndarray, np.ndarray, Line2D]: ...
def csd(
Expand Down
12 changes: 6 additions & 6 deletions lib/matplotlib/mlab.py
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Original file line number Diff line number Diff line change
Expand Up @@ -363,8 +363,8 @@ def _spectral_helper(x, y=None, NFFT=None, Fs=None, detrend_func=None,
result[slc] *= scaling_factor

# MATLAB divides by the sampling frequency so that density function
# has units of dB/Hz and can be integrated by the plotted frequency
# values. Perform the same scaling here.
# has units of V**2/Hz, if x is measured in units of V and the sampling

@rcomer rcomer Aug 17, 2026

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I'm not familiar with either this code or MATLAB, so finding this a little confusing. Do we have V**2 here or dB?

# frequency is measured in Hz. Perform the same scaling here.
if scale_by_freq:
result /= Fs
# Scale the spectrum by the norm of the window to compensate for
Expand Down Expand Up @@ -470,10 +470,10 @@ def _single_spectrum_helper(
`.detrend_mean`. 'linear' calls `.detrend_linear`.

scale_by_freq : bool, default: True
Whether the resulting density values should be scaled by the scaling
frequency, which gives density in units of 1/Hz. This allows for
integration over the returned frequency values. The default is True for
MATLAB compatibility.""")
Whether the resulting density values should be divided by the sampling
frequency, which gives density in units of 1/Hz, if the sampling rate
is measured in Hz. This allows for integration over the returned
frequency values. The default is True for MATLAB compatibility.""")


@_docstring.interpd
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2 changes: 2 additions & 0 deletions lib/matplotlib/pyplot.py
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Original file line number Diff line number Diff line change
Expand Up @@ -4070,6 +4070,7 @@ def psd(
sides: Literal["default", "onesided", "twosided"] | None = None,
scale_by_freq: bool | None = None,
return_line: bool | None = None,
Funits: str | None = None,
*,
data: DataParamType = None,
**kwargs,
Expand All @@ -4086,6 +4087,7 @@ def psd(
sides=sides,
scale_by_freq=scale_by_freq,
return_line=return_line,
Funits=Funits,
**({"data": data} if data is not None else {}),
**kwargs,
)
Expand Down
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