AngusTheMan AngusTheMan - 3 years ago 272
Python Question

Non-linear regression in Seaborn Python

I have the following dataframe that I wish to perform some regression on. I am using Seaborn but can't quite seem to find a non-linear function that fits. Below is my code and it's output, and below that is the dataframe I am using, df. Note I have truncated the axis in this plot.

I would like to fit either a Poisson or Gaussian distribution style of function.

import pandas
import seaborn


graph = seaborn.lmplot('$R$', 'Equilibrium Value', data = df, fit_reg=True, order=2, ci=None)
graph.set(xlim = (-0.25,10))


However this produces the following figure.

enter image description here

df

R Equilibrium Value
0 5.102041 7.849315e-03
1 4.081633 2.593005e-02
2 0.000000 9.990000e-01
3 30.612245 4.197446e-14
4 14.285714 6.730133e-07
5 12.244898 5.268202e-06
6 15.306122 2.403316e-07
7 39.795918 3.292955e-18
8 19.387755 3.875505e-09
9 45.918367 5.731842e-21
10 1.020408 9.936863e-01
11 50.000000 8.102142e-23
12 2.040816 7.647420e-01
13 48.979592 2.353931e-22
14 43.877551 4.787156e-20
15 34.693878 6.357120e-16
16 27.551020 9.610208e-13
17 29.591837 1.193193e-13
18 31.632653 1.474959e-14
19 3.061224 1.200807e-01
20 23.469388 6.153965e-11
21 33.673469 1.815181e-15
22 42.857143 1.381050e-19
23 25.510204 7.706746e-12
24 13.265306 1.883431e-06
25 9.183673 1.154141e-04
26 41.836735 3.979575e-19
27 36.734694 7.770915e-17
28 18.367347 1.089037e-08
29 44.897959 1.657448e-20
30 16.326531 8.575577e-08
31 28.571429 3.388120e-13
32 40.816327 1.145412e-18
33 11.224490 1.473268e-05
34 24.489796 2.178927e-11
35 21.428571 4.893541e-10
36 32.653061 5.177167e-15
37 8.163265 3.241799e-04
38 22.448980 1.736254e-10
39 46.938776 1.979881e-21
40 47.959184 6.830820e-22
41 26.530612 2.722925e-12
42 38.775510 9.456077e-18
43 6.122449 2.632851e-03
44 37.755102 2.712309e-17
45 10.204082 4.121137e-05
46 35.714286 2.223883e-16
47 20.408163 1.377819e-09
48 17.346939 3.057373e-08
49 7.142857 9.167507e-04


EDIT

Attached are two graphs produced from both this and another data set when increasing the order parameter beyond 20.

enter image description here

enter image description here

Order = 3 enter image description here

Answer Source

I have problems understanding why a lmplot is needed here. Usually you want to perform a fit by taking a model function and fit it to the data. Assume you want a gaussian function

model = lambda x, A, x0, sigma, offset:  offset+A*np.exp(-((x-x0)/sigma)**2)

you can fit it to your data with scipy.optimize.curve_fit:

popt, pcov = curve_fit(model, df["R"].values, 
                              df["EquilibriumValue"].values, p0=[1,0,2,0])

Complete code:

import pandas as pd
import numpy as np
from scipy.optimize import curve_fit
import matplotlib.pyplot as plt

df = ... # your dataframe

# plot data
plt.scatter(df["R"].values,df["EquilibriumValue"].values, label="data")

# Fitting
model = lambda x, A, x0, sigma, offset:  offset+A*np.exp(-((x-x0)/sigma)**2)
popt, pcov = curve_fit(model, df["R"].values, 
                              df["EquilibriumValue"].values, p0=[1,0,2,0])
#plot fit
x = np.linspace(df["R"].values.min(),df["R"].values.max(),250)
plt.plot(x,model(x,*popt), label="fit")

# Fitting
model2 = lambda x, sigma:  model(x,1,0,sigma,0)
popt2, pcov2 = curve_fit(model2, df["R"].values, 
                              df["EquilibriumValue"].values, p0=[2])
#plot fit2
x2 = np.linspace(df["R"].values.min(),df["R"].values.max(),250)
plt.plot(x2,model2(x2,*popt2), label="fit2")

plt.xlim(None,10)
plt.legend()
plt.show()

enter image description here

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