This report is not useful without further information and a sample script showing the problem.
For what it's worth, the usual difficulty with fitting to time/date data is that the data points are very far from the origin (time 0 = epoch date = is 1 Jan 1970). Generally you want to fit a relative time (offset from start of data) rather than an absolute time.
If you would like to refer to this comment somewhere else in this project, copy and paste the following link:
That's true, my time/date data are of Apr 2022. Trying with just the day (from day 2 to 29) works just fine. Still... maybe gnuplot could automatically subtract the first x value to make the fit work ? (that would simplify things a lot). My data are of the kind
02/04/2022 -3,3
03/04/2022 4,7
04/04/2022 2,5
05/04/2022 -1,2
06/04/2022 1,0
etc ; here is the result for the fitting function f(x) = r*x + z:
iter chisq delta/lim lambda r z
0 7.8958946412e+19 0.00e+00 1.17e+09 1.000000e+00 1.000000e+00
1 2.2682834361e+16 -3.48e+08 1.17e+08 1.694915e-02 1.000000e+00
2 6.7404943714e+08 -3.37e+12 1.17e+07 2.921210e-06 1.000000e+00
3 2.4947496954e+02 -2.70e+11 1.17e+06 -5.487712e-10 1.000000e+00
4 2.4947296583e+02 -8.03e-01 1.17e+05 -5.538087e-10 1.000000e+00
iter chisq delta/lim lambda r z
After 4 iterations the fit converged.
final sum of squares of residuals : 249.473
rel. change during last iteration : -8.03176e-06
degrees of freedom (FIT_NDF) : 27
rms of residuals (FIT_STDFIT) = sqrt(WSSR/ndf) : 3.03969
variance of residuals (reduced chisquare) = WSSR/ndf : 9.23974
Final set of parameters Asymptotic Standard Error
======================= ==========================
r = -5.53809e-10 +/- 7.809e-07 (1.41e+05%)
z = 1 +/- 1288 (1.288e+05%)
correlation matrix of the fit parameters:
r z
r 1.000
z -1.000 1.000
[When plotting just the day (as xdata), I obtain reasonable values: r=0.0269 and z = -0.17]
Last edit: Justin Sales 2022-12-22
If you would like to refer to this comment somewhere else in this project, copy and paste the following link:
This report is not useful without further information and a sample script showing the problem.
For what it's worth, the usual difficulty with fitting to time/date data is that the data points are very far from the origin (time 0 = epoch date = is 1 Jan 1970). Generally you want to fit a relative time (offset from start of data) rather than an absolute time.
That's true, my time/date data are of Apr 2022. Trying with just the day (from day 2 to 29) works just fine. Still... maybe gnuplot could automatically subtract the first x value to make the fit work ? (that would simplify things a lot). My data are of the kind
02/04/2022 -3,3
03/04/2022 4,7
04/04/2022 2,5
05/04/2022 -1,2
06/04/2022 1,0
etc ; here is the result for the fitting function f(x) = r*x + z:
iter chisq delta/lim lambda r z
0 7.8958946412e+19 0.00e+00 1.17e+09 1.000000e+00 1.000000e+00
1 2.2682834361e+16 -3.48e+08 1.17e+08 1.694915e-02 1.000000e+00
2 6.7404943714e+08 -3.37e+12 1.17e+07 2.921210e-06 1.000000e+00
3 2.4947496954e+02 -2.70e+11 1.17e+06 -5.487712e-10 1.000000e+00
4 2.4947296583e+02 -8.03e-01 1.17e+05 -5.538087e-10 1.000000e+00
iter chisq delta/lim lambda r z
After 4 iterations the fit converged.
final sum of squares of residuals : 249.473
rel. change during last iteration : -8.03176e-06
degrees of freedom (FIT_NDF) : 27
rms of residuals (FIT_STDFIT) = sqrt(WSSR/ndf) : 3.03969
variance of residuals (reduced chisquare) = WSSR/ndf : 9.23974
Final set of parameters Asymptotic Standard Error
======================= ==========================
r = -5.53809e-10 +/- 7.809e-07 (1.41e+05%)
z = 1 +/- 1288 (1.288e+05%)
correlation matrix of the fit parameters:
r z
r 1.000
z -1.000 1.000
[When plotting just the day (as xdata), I obtain reasonable values: r=0.0269 and z = -0.17]
Last edit: Justin Sales 2022-12-22
Right, so instead of fitting f(time) = r·time+z , you define a suitable time 0 and fit the offset: