Plot a horizontal line using matplotlib

Question

I have used spline interpolation to smooth a time series and would also like to add a horizontal line to the plot  But there seems to be an issue that is out of my grips  Any assistance would be really helpful  Here is what I have   annual   np arange 1 21 1  l   np array value list    a list with 20 values spl   UnivariateSpline annual l  xs   np linspace 1 21 200  plt plot xs spl xs   b    plt plot  0 len xs    40 40   r--  lw 2  pylab ylim  0 200   plt show     problem seems to be with my use of  0 len xs   for horizontal line plotting

User · Answer

If you want to draw a horizontal line in the axes, you might also try ax.hlines() method. You need to specify y position and xmin and xmax in the data coordinate (i.e, your actual data range in the x-axis). A sample code snippet is:

import matplotlib.pyplot as plt
import numpy as np

x = np.linspace(1, 21, 200)
y = np.exp(-x)

fig, ax = plt.subplots()
ax.plot(x, y)
ax.hlines(y=0.2, xmin=4, xmax=20, linewidth=2, color='r')

plt.show()

The snippet above will plot a horizontal line in the axes at y=0.2. The horizontal line starts at x=4 and ends at x=20. The generated image is:

User · Answer

A nice and easy way for those people who always forget the command axhline is the following  plt plot x   y  len x     In your case xs   x and y   40  If len x  is large  then this becomes inefficient and you should really use axhline

User · Answer

You can use plt grid to draw a horizontal line   import numpy as np from matplotlib import pyplot as plt from scipy interpolate import UnivariateSpline from matplotlib ticker import LinearLocator    your data here annual   np arange 1 21 1  l   np random random 20  spl   UnivariateSpline annual l  xs   np linspace 1 21 200     plot your data plt plot xs spl xs   b      horizental line  ax   plt axes     three ticks  ax yaxis set major locator LinearLocator 3     plot grids only on y axis on major locations plt grid True  which  major   axis  y      show plt show

User · Answer

You re looking for axhline  a horizontal axis line   For example  the following will give you a horizontal line at y   0 5   import matplotlib pyplot as plt plt axhline y 0 5  color  r   linestyle  -   plt show

User · Answer

In addition to the most upvoted answer here  one can also chain axhline after calling plot on a pandas s DataFrame   import pandas as pd   pd DataFrame  1  2  3       plot kind  bar   color  orange       axhline y 1 5

User · Answer

Use matplotlib pyplot hlines   Plot multiple horizontal lines by passing a list to the y parameter  y can be passed as a single location  y 40 y can be passed as multiple locations  y  39  40  41  If you re a plotting a figure with something like fig  ax   plt subplots    then replace plt hlines or plt axhline with ax hlines or ax axhline  respectively  matplotlib pyplot axhline can only plot a single location  e g  y 40   plt plot import numpy as np import matplotlib pyplot as plt  xs   np linspace 1  21  200   plt figure figsize  6  3   plt hlines y 39 5  xmin 100  xmax 175  colors  aqua   linestyles  -   lw 2  label  Single Short Line   plt hlines y  39  40  41   xmin  0  25  50   xmax  len xs    colors  purple   linestyles  --   lw 2  label  Multiple Lines   plt legend bbox to anchor  1 04 0 5   loc  quot center left quot   borderaxespad 0    ax plot import numpy as np import matplotlib pyplot as plt  xs   np linspace 1  21  200  fig   ax1  ax2    plt subplots 2  1  figsize  6  6    ax1 hlines y 40  xmin 0  xmax len xs   colors  r   linestyles  --   lw 2  ax1 set title  One Line    ax2 hlines y  39  40  41   xmin 0  xmax len xs   colors  purple   linestyles  --   lw 2  ax2 set title  Multiple Lines    plt tight layout   plt show     Time Series Axis  xmin and xmax will accept a date like  2020-09-10  or datetime 2020  9  10   xmin datetime 2020  9  10   xmax datetime 2020  9  10    timedelta days 3  Given date   df index 9   xmin date  xmax date   pd Timedelta days 3   where the index is a DatetimeIndex     import pandas datareader as web    conda or pip install this  not part of pandas import pandas as pd import matplotlib pyplot as plt    get test data df   web DataReader   gspc   data source  yahoo   start  2020-09-01   end  2020-09-28   iloc     2     plot dataframe ax   df plot figsize  9  6   title  S amp P 500   ylabel  Price      add horizontal line ax hlines y 3450  xmin  2020-09-10   xmax  2020-09-17   color  purple   label  test    ax legend   plt show      Sample time series data if web DataReader doesn t work   data    pd Timestamp  2020-09-01 00 00 00      High   3528 03   Low   3494 6   pd Timestamp  2020-09-02 00 00 00      High   3588 11   Low   3535 23   pd Timestamp  2020-09-03 00 00 00      High   3564 85   Low   3427 41   pd Timestamp  2020-09-04 00 00 00      High   3479 15   Low   3349 63   pd Timestamp  2020-09-08 00 00 00      High   3379 97   Low   3329 27   pd Timestamp  2020-09-09 00 00 00      High   3424 77   Low   3366 84   pd Timestamp  2020-09-10 00 00 00      High   3425 55   Low   3329 25   pd Timestamp  2020-09-11 00 00 00      High   3368 95   Low   3310 47   pd Timestamp  2020-09-14 00 00 00      High   3402 93   Low   3363 56   pd Timestamp  2020-09-15 00 00 00      High   3419 48   Low   3389 25   pd Timestamp  2020-09-16 00 00 00      High   3428 92   Low   3384 45   pd Timestamp  2020-09-17 00 00 00      High   3375 17   Low   3328 82   pd Timestamp  2020-09-18 00 00 00      High   3362 27   Low   3292 4   pd Timestamp  2020-09-21 00 00 00      High   3285 57   Low   3229 1   pd Timestamp  2020-09-22 00 00 00      High   3320 31   Low   3270 95   pd Timestamp  2020-09-23 00 00 00      High   3323 35   Low   3232 57   pd Timestamp  2020-09-24 00 00 00      High   3278 7   Low   3209 45   pd Timestamp  2020-09-25 00 00 00      High   3306 88   Low   3228 44   pd Timestamp  2020-09-28 00 00 00      High   3360 74   Low   3332 91    df   pd DataFrame from dict data   index

User · Answer

You are correct  I think the  0 len xs   is throwing you off  You ll want to reuse the original x-axis variable xs and plot that with another numpy array of the same length that has your variable in it   annual   np arange 1 21 1  l   np array value list    a list with 20 values spl   UnivariateSpline annual l  xs   np linspace 1 21 200  plt plot xs spl xs   b         horizontal line horiz line data   np array  40 for i in xrange len xs     plt plot xs  horiz line data   r--               plt plot  0 len xs    40 40   r--  lw 2  pylab ylim  0 200   plt show     Hopefully that fixes the problem

[python] Plot a horizontal line using matplotlib

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